Revert registerUserKeys back to a synchronous API

[winterhazel]

Description

#9504 made the registerUserKeys API asynchronous without a valid reason. This PR reverts it to a synchronous API, in order to avoid changing how it is consumed.

Additionally, the deleteUserKeys API, introduced in #9504, was also made synchronous, as there's no reason for it being asynchronous.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

• I registered a few API key pairs via the UI, and verified that it still works.
• I deleted a few API key pairs via the UI, and verified that it also still works.

[winterhazel]

@blueorangutan package

[blueorangutan]

@winterhazel a [SL] Jenkins job has been kicked to build packages. It will be bundled with no SystemVM templates. I'll keep you posted as I make progress.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.65%. Comparing base (dbee809) to head (8155924).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tack/api/command/admin/user/DeleteUserKeysCmd.java	0.00%	1 Missing ⚠️
...ck/api/command/admin/user/RegisterUserKeysCmd.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #13896   +/-   ##
=========================================
  Coverage     19.64%   19.65%           
- Complexity    19784    19794   +10     
=========================================
  Files          6368     6368           
  Lines        574889   574859   -30     
  Branches      70353    70350    -3     
=========================================
+ Hits         112935   112984   +49     
+ Misses       449684   449602   -82     
- Partials      12270    12273    +3     

Flag	Coverage Δ
uitests	3.41% <ø> (+<0.01%)	⬆️
unittests	20.92% <0.00%> (+0.01%)	⬆️

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

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[blueorangutan]

Packaging result [SF]: ✔️ el8 ✔️ el9 ✔️ el10 ✔️ debian ✔️ suse15. SL-JID 18874

[weizhouapache]

@blueorangutan test

[blueorangutan]

@weizhouapache a [SL] Trillian-Jenkins test job (ol8 mgmt + kvm-ol8) has been kicked to run smoke tests