[Draft][Splicing] Add reserve check to splicing

[optout21]

This is a continuation of #3407, adds proper channel balance/reserve check, when handling splice_ack (on initiator side).

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[optout21]

Relevant comments from #3407:

• value_to_self_msat doesn't include fees and anchor outputs value, to be considered similar to how it's done in build_commitment_transaction
  [Splicing] Partial, handle splice_init & splice_ack messages #3407 (comment)

• [Splicing] Partial, handle splice_init & splice_ack messages #3407 (comment)

• [Splicing] Partial, handle splice_init & splice_ack messages #3407 (comment)

Relevant method: check_splice_balances_meet_v2_reserve_requirements

[optout21]

Suggestions implemented; for reserve requirement check, the balance is adjusted with fees and eventual anchor output value (see 6ec1a19).
On hold, waiting for #3407 to land first.

[optout21]

Rebased, post #3407. (TODO: review commits, first 2 probably outdated)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.23%. Comparing base (4c43a5b) to head (8ac1467).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3641      +/-   ##
==========================================
+ Coverage   89.18%   89.23%   +0.05%     
==========================================
  Files         155      155              
  Lines      119274   119274              
  Branches   119274   119274              
==========================================
+ Hits       106379   106439      +60     
+ Misses      10290    10240      -50     
+ Partials     2605     2595      -10     

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