Provide Built Counterparty Commitment Transactions To ChannelMonitor

[tankyleo]

Provide built counterparty commitment transactions to `ChannelMonitor`

`LatestCounterpartyCommitmentTXInfo` provides `ChannelMonitor` with the
data it needs to build counterparty commitment transactions.
`ChannelMonitor` then rebuilds commitment transactions from these pieces
of data when requested.

This commit adds a new variant to `ChannelMonitorUpdateStep` called
`LatestCounterpartyCommitmentTX`, which will provide fully built
commitment transactions to `ChannelMonitor`. When `ChannelMonitor` is
asked for counterparty commitment transactions, it will not rebuild
them, but just clone them.

As a result, this new variant eliminates any calls to the upcoming
`TxBuilder` trait in `ChannelMonitor`, which avoids adding a generic
parameter on `ChannelMonitor`. This commit also stomps any bugs that
might come from passing around disparate pieces of data to re-build
commitment transactions from scratch.

We also add a `htlc_outputs` field to the variant to include the dust
HTLCs, as well as all the HTLC sources. This means that this new variant
will store non-dust HTLCs both in the `htlc_outputs` field, and in the
`commitment_tx` field. This is ok for now as the number of HTLCs
per commitment nowadays is very low.

We add code to handle this new variant, but refrain from
immediately setting it. This will allow us to atomically switch from
`LatestCounterpartyCommitmentTXInfo` to `LatestCounterpartyCommitmentTX`
in the future while still allowing downgrades.

This was previously part of #3606

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

Attention: Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.23%. Comparing base (4c43a5b) to head (29a8b9f).

Files with missing lines	Patch %	Lines
lightning/src/chain/channelmonitor.rs	81.57%	7 Missing ⚠️
lightning/src/ln/channel.rs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3654      +/-   ##
==========================================
+ Coverage   89.18%   89.23%   +0.04%     
==========================================
  Files         155      155              
  Lines      119274   119280       +6     
  Branches   119274   119280       +6     
==========================================
+ Hits       106379   106440      +61     
+ Misses      10290    10241      -49     
+ Partials     2605     2599       -6     

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