Introduce Padding for Payment and Message Blinded Tlvs

[shaavan]

Description

This PR introduces padding for Payment and Message Blinded TLVs to ensure that the size of each packet in the path is uniform.

Notes:

1. Padding Implementation:
   • The padding field is now expanded to contain Option<Vec>. This change allows the inclusion of padding within the packet while retaining the flexibility to leave the padding as None.
2. Impact on Path Length:
   • With the added padding, the length of each individual packet increases, which results in a reduction of the maximum path length. Consequently, the max_path_length in forward_check_failures has been adjusted from 18 to 17.

Codecov Report

Attention: Patch coverage is 98.48485% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.53%. Comparing base (1434e9c) to head (758d06d).

Files with missing lines	Patch %	Lines
lightning/src/blinded_path/utils.rs	94.44%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #3177 +/- ##
==========================================
- Coverage 88.54% 88.53% -0.01% 
==========================================
Files 149 149 Lines 114579 114704 +125 Branches 114579 114704 +125 ==========================================
+ Hits 101457 101558 +101 - Misses 10633 10650 +17 - Partials 2489 2496 +7 

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Updated from pr3177.01 to pr3177.02 (diff):
Addressed @jkczyz's comments.

Changes:

1. Reordered commits and separated the update to the Padding struct into a separate commit.
2. Updated the Padding struct to contain a usize instead of a Vec to save on heap allocation, and made the Writable implementation more efficient.
3. Introduced two f: commits to use iter clone instead of Vec allocation, keeping the code efficient.
4. Introduced a test and a test utility to verify that blinded message and payment paths are properly padded, ensuring a consistent payload size.

Updated from pr3177.02 to pr3177.03 (diff):

Changes:

1. Rebase on main to fix ci.

Diff post rebase:

--- a/lightning/src/ln/offers_tests.rs+++ b/lightning/src/ln/offers_tests.rs@@ -1789,7 +1789,7 @@ fn test_blinded_path_padding() {
let onion_message = charlie.onion_messenger.next_onion_message_for_peer(david_id).unwrap();
david.onion_messenger.handle_onion_message(&charlie_id, &onion_message);
- let invoice = extract_invoice(david, &onion_message);+ let (invoice, _) = extract_invoice(david, &onion_message);
assert_eq!(invoice, expected_invoice);

Updated from pr3177.03 to pr3177.04 (diff):
Addressed @jkczyz comments

1. Refactor max_length calculation to be cleaner.
2. Introduce wrapper struct instead of tuple to make the code clearer to understand.
3. Move the padding testing to onion_message/functional_tests.rs to simplify the setup, and test the code at a more appropriate place.

Updated from pr3177.04 to pr3177.05 (diff):

Updates:

1. Rebase on main.

Updated from pr3177.05 to pr3177.06 (diff):
Addressed @jkczyz comments

Changes:

1. Squash commits.
2. Introduce a generic struct WithPadding to keep the code DRY.
3. Introduce a test for Blinded Payment Paths.
4. Use saturating sub in padding length calculation to avoid overflows.

Updated from pr3177.06 to pr3177.07 (diff):
Addressed @jkczyz comments

1. Update comments.
2. Introduce debug_assert!() in Writeable for WithPadding.
3. Properly assert!() the condition in test.

Updated from pr3177.07 to pr3177.08 (diff):
Addressed @jkczyz comments

Changes:

1. Introduce an f: commit with an alternative approach to WithPadding.
2. The new approach introduces padding as an optional field within the ForwardTlvs and the ReceiveTlvs struct.

Update: From pr3177.08 to pr3177.09 (diff):

Changes based on @jkczyz's feedback:

1. Fixed the imports.
2. Removed BlindedPaymentTlvsRef and replaced it with BlindedPaymentTlvs. The former contained an immutable reference to the underlying TLVs, but the new implementation required mutability. Therefore, I transitioned to using BlindedPaymentTlvs directly, making BlindedPaymentTlvsRef redundant.

@TheBlueMatt One thing that I'm unsure of is whether we only need to pad the blinded path or if we should also pad the unblinded portion of the onion.

Update: From pr3177.09 to pr3177.10 (diff):

Changes based on @jkczyz's feedback:

1. Rename pad_tlvs -> pad_to_length.

Update: From pr3177.29 to pr3177.30 (diff):
Addressed @jkczyz comments

Changes:

1. Updated the comment on the reasoning behind not reading padding.
2. Update TLV_OVERHEAD to only account for padding type and length bytes.
3. Reverted changes to the is_padded function.

Update: From pr3177.30 to pr3177.31 (diff):

Changes:

1. Rebase on main to resolve to merge conflicts.

Update: From pr3177.31 to pr3177.32 (diff):

Changes:

1. Squash commits, and expands commit message to make their purpose clearer.

Update: From pr3177.32 to pr3177.33 (diff):

Changes:

1. Rebase on main, to resolve merge conflicts.

LGTM, @jkczyz you wanna take another look?

Looks like rustfmt CI checks are failing. Otherwise, just one nit and LGTM.

Update: From pr3177.33 to pr3177.34 (diff):
Addressed @jkczyz comments:

1. Fix rust formatting
2. Addressed nit suggestion

Update: From pr3177.34 to pr3177.35 (diff):

Changes:

1. Rebase on main, resolving merge conflicts.