Add auth delegation utilities

[MikailBag]

Motivation

Applications usually perform some authentication & authorization, and for kubernetes-native apps it makes sense to leverage apiserver for that, e.g. this is recommended for the extension API servers.

Solution

This PR adds simple high-level client together with SubjectAccessReview builders.

Additionally, while I was writing an example, I realized that currently it is not possible to create subresources, so I added a method for this in a separate commit.

[codecov-commenter]

Codecov Report

Merging #778 (647233e) into master (4d991fe) will decrease coverage by 1.69%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
- Coverage   71.99%   70.29%   -1.70%     
==========================================
  Files          54       55       +1     
  Lines        3631     3720      +89     
==========================================
+ Hits         2614     2615       +1     
- Misses       1017     1105      +88     

Impacted Files	Coverage Δ
kube-client/src/lib.rs	92.80% <ø> (ø)
kube-client/src/util/auth.rs	0.00% <0.00%> (ø)
kube-core/src/request.rs	92.16% <0.00%> (-2.84%)	⬇️
kube/src/lib.rs	87.80% <ø> (ø)
kube-runtime/src/wait.rs	70.00% <0.00%> (+2.00%)	⬆️

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[clux]

Hey, thanks a lot for all of this! I think this makes sense to include a lot of this in kube, and will come back in a day or two to give this a proper review (the tokenreview stuff is all new to me, so need to read through this properly).

Some quick notes:

• extra subresource create absolutely makes sense
• authclient and associated enums makes sense
• the builders for subjectaccessreview possibly makes sense - they might clash with optional builder patterns k8s-pb#9 down the line though 🤔

[MikailBag]

I'm going to close this:
builders, while being useful, are probably out-of-scope for kube-rs, and the rest is already implemented in other PRs