[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.