7. feat(state): add a read-only state service stub

[teor2345]

Motivation

In #3745, we want to add a read-only, cloneable state service, with no buffer or queue.

Designs

Share access to the best non-finalized chain and the finalized state.

Solution

• Create a ReadStateService struct
  • Add a stub service implementation
  • Add support methods in the finalized and non-finalized state
• Make RPC accept a buffered state or a read-only state
• Doc comment updates

Review

There is a lot of code changed in this PR series, it would be good to get it merged soon to avoid conflicts.

This doesn't do anything yet, the next few PRs will implement the read-only state requests, and add tests.

Reviewer Checklist

• Code implements Specs and Designs

Follow Up Work

• implement block requests
• use in the RPC implementation
• test performance
• add tests

[codecov]

Codecov Report

Merging #3835 (fe60415) into main (f39ac48) will decrease coverage by 0.16%.
The diff coverage is 65.82%.

@@            Coverage Diff             @@
##             main    #3835      +/-   ##
==========================================
- Coverage   78.82%   78.65%   -0.17%     
==========================================
  Files         293      293              
  Lines       33690    33737      +47     
==========================================
- Hits        26555    26535      -20     
- Misses       7135     7202      +67     

[oxarbitrage]

Looks good, lets merge.