refactor(remote STS): lazy fetch the secret from the vault before request

[wolf4ood]

What this PR changes/adds

The StsRemoteClientConfiguration now holds the secret alias instead of the secret itself.

The RemoteSecureTokenService before sending the request fetches the secret from the vault with the configured secret alias.

Linked Issue(s)

Closes #4412

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.96%. Comparing base (7f20ba5) to head (7639fad).
Report is 386 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4413      +/-   ##
==========================================
+ Coverage   71.74%   74.96%   +3.21%     
==========================================
  Files         919     1073     +154     
  Lines       18457    21462    +3005     
  Branches     1037     1172     +135     
==========================================
+ Hits        13242    16089    +2847     
- Misses       4756     4852      +96     
- Partials      459      521      +62     

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