Handle GitHub Rate Limits During Update Checks

[rgajason]

GGShield checks for updates by connecting to GitHub anonymously. The anonymous API quota is relatively small. For organizations where hundreds of users all connect via a common egress IP, this can result in significant error messages (collectively across users) / confusion during GGShield operations. For example:

% git commit -m 'My Awesome Commit' awesome.py
Failed to check: {"message":"API rate limit exceeded for 1.2.3.4. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}

This PR address this issue by checking for quota exceeded condition and rescheduling the update check for after the quota has been reset.

Additionally, it adds headers to the update check request that are recommended by GitHub:

User Agent: https://docs.github.com/en/rest/overview/resources-in-the-rest-api?apiVersion=2022-11-28#user-agent-required
GH API Version: https://docs.github.com/en/rest/overview/api-versions?apiVersion=2022-11-28
Media type (accept): https://docs.github.com/en/rest/overview/media-types?apiVersion=2022-11-28

[codecov-commenter]

Codecov Report

Merging #449 (019dd91) into main (4d81348) will decrease coverage by 0.26%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #449      +/-   ##
==========================================
- Coverage   93.89%   93.63%   -0.27%     
==========================================
  Files          78       78              
  Lines        3557     3567      +10     
==========================================
  Hits         3340     3340              
- Misses        217      227      +10     

Flag	Coverage Δ
unittests	93.63% <0.00%> (-0.27%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
ggshield/core/check_updates.py	68.85% <0.00%> (-13.51%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[agateau-gg]

Two minor remarks, but this looks great already. Thanks a lot for taking care of the bug and for adding the missing headers to the request.

[rgajason]

@agateau-gg both changes implemented as requested

[agateau-gg]

Thanks, merging the changes in.