Infinite default timeout for WhoAmI API call and add --timeout CLI flag

[cfryanr]

The pinniped whoami CLI command has always had a hardcoded timeout of 20 seconds for the WhoAmI API call. However, this has always been a problem. When a user needs to interactively authenticate, and when they take more than 20 seconds to do so, then the WhoAmI request always fails due to the timeout being exceeded, even though they successfully logged in.

To fix this, copy how kubectl timeouts work. Introduce a new --timeout flag (name of flag copied from the pre-existing pinniped get kubeconfg flag) and default it to 0 which means no timeout (i.e. infinite timeout).

The fake Kube client used in the unit tests ignores the context param that is passed to it, so unfortunately there is no easy way to unit test this timeout.

Release note:

`pinniped whoami` has a new `--timeout` parameter, which defaults to no timeout.

[joshuatcasey]

Approved, check review comments.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (61c630c) 78.96% compared to head (47f6de5) 78.94%.

Files	Patch %	Lines
cmd/pinniped/cmd/whoami.go	28.57%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1774      +/-   ##
==========================================
- Coverage   78.96%   78.94%   -0.03%     
==========================================
  Files         166      166              
  Lines       15933    15938       +5     
==========================================
  Hits        12582    12582              
- Misses       3036     3040       +4     
- Partials      315      316       +1     

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