Update kubectl example to include optional context argument

[mgs255]

Allow overriding current context with the named context from the user's kubeconfig file.

Motivation

Small extension to the kubectl example, mirroring the functionality provided by the official kubectl tool.
There is not a similar example in the examples codebase of using the KubeConfigOptions.

Solution

Switch from the existing Client::try_default().await? implementation to one that is hopefully equivalent and also supports optionally specifying a named context.

[clux]

this looks sensible to me. kind of highlights that perhaps KubeConfigOptions should have a builder so that these 4 lines becomes would become let options = KubeConfigOptions::default().context(app.context);.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.6%. Comparing base (8b5230f) to head (c9b9ab5).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1647     +/-   ##
=======================================
+ Coverage   75.3%   75.6%   +0.3%     
=======================================
  Files         82      82             
  Lines       7371    7405     +34     
=======================================
+ Hits        5550    5591     +41     
+ Misses      1821    1814      -7     

see 7 files with indirect coverage changes

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