throttler (v22): towards formal gRPC calls in endtoend tests, removing HTTP API calls

[shlomi-noach]

Description

For v22

This PR removes any references to /throttler/* API calls, and uses gRPC instead.

This is to be done in v22 (right after v21 is released) because some endtoend tests and functionality is used in upgrade/downgrade testing and needs to keep supporting HTTP endpoints.

Related Issue(s)

#15624

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.48%. Comparing base (cb66920) to head (cdf86f5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16530      +/-   ##
==========================================
- Coverage   67.49%   67.48%   -0.01%     
==========================================
  Files        1577     1577              
  Lines      253416   253374      -42     
==========================================
- Hits       171045   170994      -51     
- Misses      82371    82380       +9     

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[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

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If no action is taken within 7 days, this PR will be closed.

[shlomi-noach]

Online DDL flow - Upgrade Downgrade Testing will only pass once release-21.0 is done.

[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.