Update splice messages according to new spec draft

[optout21]

Some changes to the splice messages according to new spec draft lightning/bolts#1160
The splicing messages are not yet used (on main).

The list of changes (some new fields, some renames):

• CommitmentSigned: New optional batch parameters, used in case multiple commitment are updated at once. Optional batch field of type CommitmentSignedBatch, containing batch size and funding txid.
• TxAddInput: New optional field shared_input_txid, to be used when during splicing the previous funding transaction is added as an input (funding_txid).
• TxSignatures: Rename optional field to shared_input_signature, from funding_outpoint_sig.
• SpliceInit: Rename msg to SpliceInit, from Splice (msg is now splice_init in spec) (also SendSpliceInit, handle_splice_init)
• SpliceInit: Rename amount field to funding_contribution_satoshis (from relative_satoshis). Note that his is the part of the initiator only.
• SpliceInit: New optional bool field require_confirmed_inputs.
• SpliceInit: The chain_hash field is removed (not needed).
• SpliceAck: Rename amount field to funding_contribution_satoshis (from relative_satoshis). Note that his is the part of the acceptor only.
• SpliceAck: New optional bool field require_confirmed_inputs.
• SpliceAck: The chain_hash field is removed (not needed).
• SpliceLocked: Added new splice_txid field, with the id of the new locked funding tx
• Change wire type for SpliceInit and SpliceAck to 80 & 81 (from 75 & 76)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.57143% with 4 lines in your changes missing coverage. Please review.

Project coverage is 91.15%. Comparing base (07f3380) to head (43a6ee3).
Report is 31 commits behind head on main.

Files	Patch %	Lines
lightning/src/ln/peer_handler.rs	0.00%	2 Missing ⚠️
lightning/src/ln/channelmanager.rs	0.00%	1 Missing ⚠️
lightning/src/ln/functional_test_utils.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3129      +/-   ##
==========================================
+ Coverage   89.92%   91.15%   +1.22%     
==========================================
  Files         121      121              
  Lines       99172   107295    +8123     
  Branches    99172   107295    +8123     
==========================================
+ Hits        89180    97803    +8623     
+ Misses       7391     7009     -382     
+ Partials     2601     2483     -118     

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[TheBlueMatt]

Didn't bother to check if it matches the spec, we'll find out when we try to use it if it doesnt.

[optout21]

Pushed some minor fuzz fixes; ready for review.

[dunxen]

LGTM. Bound to be some additional changes coming up anyway, but I've reviewed what's here against the current draft messages.