Initial WIP on CustomInfo for agents

[ddl-ebrown]

Goes with flyteorg/flytekit#2426

I was able to get the agent sending back CustomInfo as part of the calls to GetTask, but unfortunately it looks like the server doesn't do anything with them. This PR aims to fix that.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.10%. Comparing base (2f1f813) to head (433d879).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5391      +/-   ##
==========================================
+ Coverage   61.08%   61.10%   +0.01%     
==========================================
  Files         793      793              
  Lines       51156    51158       +2     
==========================================
+ Hits        31251    31259       +8     
+ Misses      17033    17027       -6     
  Partials     2872     2872              

Flag	Coverage Δ
unittests-datacatalog	69.31% <ø> (ø)
unittests-flyteadmin	58.90% <ø> (+0.04%)	⬆️
unittests-flytecopilot	17.79% <ø> (ø)
unittests-flytectl	68.31% <ø> (ø)
unittests-flyteidl	79.30% <ø> (ø)
unittests-flyteplugins	61.95% <100.00%> (+<0.01%)	⬆️
unittests-flytepropeller	57.32% <ø> (ø)
unittests-flytestdlib	65.75% <ø> (ø)

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