refactor: remove op codes from btc vault swap encoding

[j4m1ef0rd]

Pull Request

Closes: PRO-1845

Checklist

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

• I am confident that the code works.
• I have written sufficient tests.
• I have written and tested required migrations.
• I have updated documentation where appropriate.

Summary

• Fixed a bug in the auto assigning of affiliate short id's in the broker api.
• Removed the op codes from the nulldata utxo.
• Renamed the nulldata_utxo -> nulldata_payload seems it's no longer a complete uxto.

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 15 lines in your changes missing coverage. Please review.

Project coverage is 72%. Comparing base (d12200a) to head (d4af5c8).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
api/lib/src/lib.rs	75%	11 Missing ⚠️
state-chain/runtime/src/runtime_apis.rs	0%	3 Missing ⚠️
state-chain/runtime/src/lib.rs	0%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5465    +/-   ##
======================================
  Coverage     72%     72%            
======================================
  Files        494     494            
  Lines      87751   87949   +198     
  Branches   87751   87949   +198     
======================================
+ Hits       62826   63010   +184     
- Misses     22310   22358    +48     
+ Partials    2615    2581    -34     

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