Improve deploy key and user key handling

[van-vliet]

NOTE:
This is a copy of #1593 made from a personal account to allow "Allow edits from maintainers". The code is similar - the commits have been rebased and squashed onto latest main though.

Description

Added additional fields to deploykey object: created_at, last_used, added_by, read_only.
Added methods to allow adding / deleting public keys on a user.

Before submitting a PR:

• Changes must not break binary backwards compatibility. If you are unclear on how to make the change you think is needed while maintaining backward compatibility, CONTRIBUTING.md for details.
• Add JavaDocs and other comments as appropriate. Consider including links in comments to relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. This generally requires capturing snapshot test data. See CONTRIBUTING.md for details.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.
• Push your changes to a branch other than main. You will create your PR from that branch.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes. This includes:
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible.
• All lines of new code should be covered by tests as reported by code coverage. Any lines that are not covered must have PR comments explaining why they cannot be covered. For example, "Reaching this particular exception is hard and is not a particular common scenario."
• Enable "Allow edits from maintainers".

[codecov]

Codecov Report

Base: 79.30% // Head: 79.50% // Increases project coverage by +0.19% 🎉

Coverage data is based on head (4f1dba4) compared to base (f48169f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1616      +/-   ##
============================================
+ Coverage     79.30%   79.50%   +0.19%     
- Complexity     2142     2154      +12     
============================================
  Files           206      206              
  Lines          6582     6601      +19     
  Branches        364      364              
============================================
+ Hits           5220     5248      +28     
+ Misses         1150     1141       -9     
  Partials        212      212              

Impacted Files	Coverage Δ
src/main/java/org/kohsuke/github/GHDeployKey.java	59.25% <100.00%> (+59.25%)	⬆️
src/main/java/org/kohsuke/github/GHKey.java	100.00% <100.00%> (ø)
src/main/java/org/kohsuke/github/GHMyself.java	45.61% <100.00%> (+6.39%)	⬆️
src/main/java/org/kohsuke/github/GHRepository.java	70.00% <0.00%> (+0.60%)	⬆️

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