api: add endpoint to list runners

[lewismiddleton]

Adds an endpoint at /groups/{name}/list to list the runners in each runner group .

Relates to #625

[lewismiddleton]

This implementation causes the encoded_jit_config to be exported as JSON. This could be a security risk in that it may allow someone with access to the runner-manager endpoint to register a runner using the same config.

I did some testing locally and I wasn't able to register a new runner using the same encoded_jit_config but I don't have high confidence that will always be the case.

[lewismiddleton]

Trying to remove the encoded_jit_config from the output in df057a2 worked in the test suite but still outputted jit_config when deployed in a live environment. Unsure why that's the case.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.52%. Comparing base (b83bf37) to head (f0c995e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
+ Coverage   86.43%   86.52%   +0.08%     
==========================================
  Files          33       33              
  Lines        1386     1395       +9     
==========================================
+ Hits         1198     1207       +9     
  Misses        188      188              

Flag	Coverage Δ
api	64.08% <100.00%> (+0.23%)	⬆️
unit	68.31% <0.00%> (-0.45%)	⬇️

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