add metadata to each client action

[shiftkey]

This PR adds attributes to each client action to provide context about where each client method is sourced from.

This comes with two main benefits:

• by including the verb and path for each route, this will help me audit what parts of the API are not implemented currently
• using ManualRouteAttribute will help me to track which code was hand-crafted, and hopefully which ones are generated by tooling in the future

I also noticed some places where things are likely broken, and some long-obsolete APIs, and I might need to make changes to this metadata down the track, but I can follow-up with other PRs later once I've given this another look.

• review diff
• extract issues for follow-up - investigate PATCH/POST mismatch in IssuesLabelsClient #2125 address potential missing method inside ProjectsClient #2126

[codecov]

Codecov Report

Merging #2124 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2124      +/-   ##
==========================================
- Coverage   66.95%   66.91%   -0.05%     
==========================================
  Files         542      543       +1     
  Lines       14240    14252      +12     
==========================================
+ Hits         9535     9537       +2     
- Misses       4705     4715      +10     

Impacted Files	Coverage Δ
Octokit/Helpers/ManualRouteAttribute.cs	0.00% <0.00%> (ø)
Octokit/SimpleJson.cs	70.41% <0.00%> (+0.26%)	⬆️

[shiftkey]

release_notes: add metadata to each client to assist with auditing API coverage