Add AChainMonitor trait and use it in background processor

[joostjager]

Add a new AChainMonitor trait following the same pattern as AChannelManager. This trait provides associated types for all generic parameters of ChainMonitor and a get_cm() method to access the underlying ChainMonitor.

Update the background processor to use AChainMonitor trait bounds instead of spelling out the full ChainMonitor generic parameters. This simplifies the function signatures by removing 5-6 explicit generic parameters (CF, T, F, P, ES) per function.

This is preparation for adding a flush method to the AChainMonitor trait.

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
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Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 75.92593% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.01%. Comparing base (35ab03f) to head (4800a47).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-background-processor/src/lib.rs	74.50%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4371      +/-   ##
==========================================
+ Coverage   85.99%   86.01%   +0.01%     
==========================================
  Files         156      156              
  Lines      102766   102781      +15     
  Branches   102766   102781      +15     
==========================================
+ Hits        88378    88409      +31     
+ Misses      11879    11864      -15     
+ Partials     2509     2508       -1     

Flag	Coverage Δ
tests	86.01% <75.92%> (+0.01%)	⬆️

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[joostjager]

Verified that this is indeed going to work with deferred writes (#4317) and ldk-node integration (lightningdevkit/ldk-node#782)

[TheBlueMatt]

Cool thanks!

[ldk-reviews-bot]

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[TheBlueMatt]

Eh, just gonna merge this even though it "broke" semver. but semver is already broken cause BP depends on lightning which are different types...