Feat: Allow using in-cluster creds in control plane cluster in a multi-cluster deployment

[fg91]

Why are the changes needed?

When configuring a Flyte multi-cluster deployment, one might want to use the control plane cluster also to schedule workflows. Currently, flyteadmin is not able to use in-cluster credentials for this use case. Instead, one has to go through the much more cumbersome credentials configuration process required for data plane clusters (see here).

This PR allows flyteadmin to just use in-cluster credentials instead.

---

As briefly discussed in the last contributors' sync @wild-endeavor.

How was this patch tested?

• A flyteadmin image with this change has been in use in our multi-cluster deployment.
• Adapted unit test.

Check all the applicable boxes

• [x] I updated the documentation accordingly.
• [ ] All new and existing tests passed.
• [x] All commits are signed-off.

Docs link

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.07%. Comparing base (16d2b14) to head (50892d1). Report is 159 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5403   +/-   ##
=======================================
  Coverage   61.07%   61.07%           
=======================================
  Files         793      793           
  Lines       51210    51210           
=======================================
+ Hits        31274    31275    +1     
+ Misses      17060    17059    -1     
  Partials     2876     2876           

Flag	Coverage Δ
unittests-datacatalog	69.31% <ø> (ø)
unittests-flyteadmin	58.90% <ø> (ø)
unittests-flytecopilot	17.79% <ø> (ø)
unittests-flytectl	67.98% <ø> (ø)
unittests-flyteidl	79.04% <ø> (ø)
unittests-flyteplugins	61.94% <ø> (ø)
unittests-flytepropeller	57.32% <ø> (ø)
unittests-flytestdlib	65.75% <ø> (+0.02%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.