add support for filesystem implementation of token cache

[nicktate]

Tracking issue

Why are the changes needed?

Using keyring service based token cache in headless environments is challenging. Many keyring implementations are tied to GUI interactions and proper headless configuration across different distros is very challenging. I propose we leave keyring as the default token cache, but enable users to opt-in for a filesystem based cache if desired.

What changes were proposed in this pull request?

• Adds a new TokenCacheTypeFilesystem implementation that may be configured via root.token-cache-type

How was this patch tested?

• Added a set of tests that mimic the functionality of the tests for token_keyring_test.go

Setup process

Screenshots

Check all the applicable boxes

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

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[codecov]

Codecov Report

Attention: Patch coverage is 60.49383% with 32 lines in your changes are missing coverage. Please review.

Project coverage is 61.10%. Comparing base (470621e) to head (da2d51e).
Report is 4 commits behind head on master.

Files	Patch %	Lines
flytectl/pkg/pkce/token_cache_filesystem.go	55.88%	17 Missing and 13 partials ⚠️
flytectl/cmd/core/cmd.go	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5421      +/-   ##
==========================================
- Coverage   61.99%   61.10%   -0.89%     
==========================================
  Files         611      794     +183     
  Lines       36375    51241   +14866     
==========================================
+ Hits        22550    31310    +8760     
- Misses      11860    17046    +5186     
- Partials     1965     2885     +920     

Flag	Coverage Δ
unittests-datacatalog	69.31% <ø> (ø)
unittests-flyteadmin	58.90% <ø> (?)
unittests-flytecopilot	17.79% <ø> (ø)
unittests-flytectl	68.18% <60.49%> (-0.14%)	⬇️
unittests-flyteidl	79.30% <ø> (ø)
unittests-flyteplugins	61.94% <ø> (ø)
unittests-flytepropeller	57.32% <ø> (ø)
unittests-flytestdlib	65.82% <ø> (+0.06%)	⬆️

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