refactor: use Client instance by reference

[2bndy5]

This moves async fn out of trait declaration to allow using Client instance passed by reference.

This avoids cloning the Client instance which can be costly to CPU and memory.

[codecov]

Codecov Report

Attention: Patch coverage is 94.39252% with 6 lines in your changes missing coverage. Please review.

Project coverage is 97.53%. Comparing base (49a7c8d) to head (1742cad).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
cpp-linter/src/rest_api/mod.rs	89.65%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
- Coverage   97.58%   97.53%   -0.05%     
==========================================
  Files          14       14              
  Lines        3430     3406      -24     
==========================================
- Hits         3347     3322      -25     
- Misses         83       84       +1     

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[2bndy5]

I think the proper way would be to use tokio's async channels, but we're not doing a massive amount of REST API work here.

[2bndy5]

The lines missing coverage were uncovered before this. Coverage went down slightly because the reformatted source has fewer total lines.