Add Api fns for arbitrary subresources and approval subresource for CertificateSigningRequest

[ChinYing-Li]

Motivation

This PR tackles two tasks in #127.

Solution

Implement Api functions for arbitrary subresource and Api approve/deny.
One test case of Api is added.
Should I try to come up with test cases for those Api subresource-related functions?
IMO, testing the logic of Request.*_subresource should be sufficient.
Any suggestion is appreciated!

[codecov-commenter]

Codecov Report

Merging #773 (16aefc7) into master (4d991fe) will decrease coverage by 0.08%.
The diff coverage is 60.86%.

@@            Coverage Diff             @@
##           master     #773      +/-   ##
==========================================
- Coverage   71.99%   71.90%   -0.09%     
==========================================
  Files          54       54              
  Lines        3631     3677      +46     
==========================================
+ Hits         2614     2644      +30     
- Misses       1017     1033      +16     

Impacted Files	Coverage Δ
kube-client/src/api/subresource.rs	55.96% <21.05%> (-7.38%)	⬇️
kube-client/src/api/util.rs	81.57% <66.66%> (-4.63%)	⬇️
kube-client/src/lib.rs	93.70% <100.00%> (+0.90%)	⬆️
kube-client/src/client/mod.rs	70.10% <0.00%> (+1.03%)	⬆️

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[clux]

hey thanks again for all these prs!

i think the general setup here is ok, but have left a few comments on the main Api<CSR> abstraction. I feel it is trying to make a few too many assumptions for what should be in kube-client, and it's probably better to keep this simple and just wrap the newly added patch_subresource method.

(however, there may be an argument here for replicating what kubectl certificate approve as a small certificate helper in kube-runtime - but that is not necessary for this pr - and not even sure we need to do it).

[clux]

This looks great now. Thanks for doing all the minor tweaks here to get it into a great state! I'll merge this in tomorrow if you are happy with it :-)

[ChinYing-Li]

I think this is ready for merge. Thanks for helping!