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

Please include a succinct description of the purpose and content of the PR. What problem does it solve, and how? Link issues, discussions, other PRs, and anything else that will help the reviewer.

Non-Breaking changes

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 100 lines in your changes missing coverage. Please review.

Project coverage is 71%. Comparing base (d4f68cc) to head (31564a9).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
state-chain/custom-rpc/src/lib.rs	0%	97 Missing ⚠️
state-chain/node/src/service.rs	0%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5456    +/-   ##
======================================
- Coverage     71%     71%    -0%     
======================================
  Files        494     494            
  Lines      87666   87642    -24     
  Branches   87666   87642    -24     
======================================
- Hits       62674   62530   -144     
- Misses     22381   22498   +117     
- Partials    2611    2614     +3     

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