feat: BTC contract swap encoding

[msgmaxim]

Pull Request

Closes: PRO-1673

Checklist

Please conduct a thorough self-review before opening the PR.

• I am confident that the code works.
• I have written sufficient tests.

Summary

• Defines UtxoEncodedData which effectively determines how swap parameters are layed out/encoded. For encoding/decoding we use Scale to keep things smiple. It still results in a reasonably compact encoding: the check_utxo_encoding test demonstrates which bytes are used for what (and that we don't exceed the limit of 80 bytes). I chose not to use "insta" in this case to make things a bit more explicit.
• SharedCfParameters defines a subset of UtxoEncodedData's fields which we expect to use in other smart contracts (this is what you intended @albert-llimos?)
• As discussed on discord, min price is encoded as min output amount to save space. The encoding of other fields is more or less straightforward.

[codecov]

Codecov Report

Attention: Patch coverage is 86.48649% with 35 lines in your changes missing coverage. Please review.

Project coverage is 71%. Comparing base (ab7104b) to head (cdde79e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
engine/src/witness/btc/smart_contract.rs	92%	12 Missing and 3 partials ⚠️
...te-chain/chains/src/btc/smart_contract_encoding.rs	78%	11 Missing and 3 partials ⚠️
state-chain/chains/src/btc.rs	60%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5311    +/-   ##
======================================
- Coverage     71%     71%    -0%     
======================================
  Files        488     490     +2     
  Lines      84898   85016   +118     
  Branches   84898   85016   +118     
======================================
+ Hits       60375   60405    +30     
- Misses     21822   21897    +75     
- Partials    2701    2714    +13     

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[albert-llimos]

SharedCfParameters defines a subset of UtxoEncodedData's fields which we expect to use in other smart contracts (this is what you intended @albert-llimos?)

Yes exactly, that looks good to me.

[albert-llimos]

The only thing left to "match" the contract swaps is the CCM parameters (message and gasAmount). I understand that we have this 80bytes limitation so it definitely doesn't fit there. I guess there might be ways around it (e.g. add another utxo for that, although still limited in length) but I assume we don't care about supporting CCM swaps, especially right now. I wouldn't care but just double checking that we're all aligned. @martin-chainflip @msgmaxim

[dandanlen]

I noticed that PushBytes allows MAX_PUSHABLE_BYTES, but we were storing it in a bounded vec of size MAX_BITCOIN_SCRIPT_LENGTH.

The only reason we defined this was to derive MaxEncodedLen, so I moved the definition of this here.

[dandanlen]

LGTM - I added one small change (see comment). Will let you double-check my commit and then push the button if you're happy.