Fix: Sanitize user identity before injecting into task pod as K8s label

[fg91]

Why are the changes needed?

@ByronHsu recently implemented middleware to inject the user identity into the flyte workflow's ExecutionSpec: flyteorg/flyteadmin#549 and in #4637 I injected the user identity into task pods as a label.

For IdPs like Okta, the execution identity is an email address which is not a valid Kubernetes label, causing the creation of the pod to fail.

What changes were proposed in this pull request?

In this PR I add sanitation to the execution identity before injecting it as a label.

How was this patch tested?

• Ran flytepropeller with this change and ensured that pods can be created when the execution identity is an email address
• Added a unit test that would have caught this

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.51%. Comparing base (992641c) to head (343c46c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5023      +/-   ##
==========================================
- Coverage   58.99%   58.51%   -0.49%     
==========================================
  Files         645      567      -78     
  Lines       55547    50933    -4614     
==========================================
- Hits        32772    29802    -2970     
+ Misses      20183    18720    -1463     
+ Partials     2592     2411     -181     

Flag	Coverage Δ
unittests	58.51% <100.00%> (-0.49%)	⬇️

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