fix: try to use ChainMap to chain settings, fix message_length_limit inconsistency

[bearomorphism]

Relate #1672

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.96%. Comparing base (de24815) to head (96caf65).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## master #1812 +/- ##
=======================================
Coverage 97.96% 97.96% =======================================
Files 60 61 +1 Lines 2648 2657 +9 =======================================
+ Hits 2594 2603 +9 
Misses 54 54 

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[woile]

Add some docstrings here:

1. What is this
2. What it does
3. How to use
4. How it compares to the previous solution

That would really help me understand the benefit.

Something I'd like to know as well:
Right now, if I make a plugin, I can create custom settings, but they are not typed. Would this allow plugin to declare their own settings?
example

[bearomorphism]

Right now, if I make a plugin, I can create custom settings, but they are not typed. Would this allow plugin to declare their own settings?

Thanks, I will put those scenarios into consideration. ChainMap approach should not break the type system.

[bearomorphism]

How it compares to the previous solution

We can discuss this in #1672. I will share the motivation there.