[Draft] Combine FundedChannel and PendingV2Channel through traits

[optout21]

The RefundingChannel -- used by splicing and possibly dual funding -- can acts as both Funded and Pending (depending on the context). To achieve that, in this PR both FundedChannel and PendingV2Channel are placed behind traits, so they can be combined easily.

This PR is more of a proof-of-concept.

Notes:

• The changes to FundedChannel methods are substantial but mostly trivial, changing fields into accessors
• There were a few cases when 2 mutable accessors would conflict (of type self.context_mut().somemethod(&self.funded())), those cases were solved by extra local ref variables or obtaining both context and funding at the same time
• One issue: the visibility differences of the methods disappeared (pub vs. non), as all trait methods are public by default. This prompted several related types to be promoted to public, which is not very nice
• The first commit carves out WithContextAndFunding from InitialRemoteCommitmentReceiver, with the original intention that PendingV2ChannelTrait would also include it. But this is not the case, as PendingV2ChannelTrait needs its own funding! So this carve-out is not really needed.

My conclusion is that this could be solved in other way:

• In RefundingChannel include a full FundedChannel, as it is accessed mostly through as_funded() methods
• Include extra fields for pending, and implement PendingV2ChannelTrait
• leave FundedChannel as it is, no need for trait there
• this would mean much less change
• see this in [Draft] RefundingChannel: combine FundedChannel and PendingV2Channel using a trait for Pending #3720

[ldk-reviews-bot]

👋 Thanks for assigning @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-reviews-bot]

🔔 1st Reminder

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Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[ldk-reviews-bot]

🔔 2nd Reminder

Hey @jkczyz! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[optout21]

Simpler version created: #3720

[ldk-reviews-bot]

🔔 3rd Reminder

Hey @jkczyz! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[jkczyz]

Left a review on #3720. Let's discuss both alternatives today.

[ldk-reviews-bot]

👋 The first review has been submitted!

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[optout21]

Closing (in favor of simpler version #3720 )