feat: custom rpc finalized block subscriptions

[nmammeri]

Pull Request

Closes: PRO-1768

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

• Adds new chain head api inside custom-rpc
• Changes subscription to use the new chainHead_v1_follow api
• Adds a new only_finalized parameter specifying whether to subscribe to finalized or best blocks
• Manual tests were conducted to test functionality

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 107 lines in your changes missing coverage. Please review.

Project coverage is 72%. Comparing base (c70d72c) to head (54de5e5).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
state-chain/custom-rpc/src/lib.rs	0%	104 Missing ⚠️
state-chain/node/src/service.rs	0%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5466    +/-   ##
======================================
- Coverage     72%     72%    -0%     
======================================
  Files        494     494            
  Lines      88073   88004    -69     
  Branches   88073   88004    -69     
======================================
- Hits       63191   63011   -180     
- Misses     22305   22413   +108     
- Partials    2577    2580     +3     

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