feat: Add MetaService.ListAPIVersions

[kyungseopk1m]

Fixes #4420

Adds MetaService.ListAPIVersions for GET /versions, operation meta/get-all-versions.

The 200 response schema is an array of string with format: date, so the method returns []string and no new type is introduced. GitignoresService.List has the same signature for the same kind of operation.

Named ListAPIVersions rather than Get... because the repo uses List for operations returning multiple resources, and MetaService holds several unrelated resources so a bare List would be ambiguous.

The operation is already in openapi_operations.yaml, so go generate ./... produces no changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (34349a8) to head (9c7e1bf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4422   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files         194      194           
  Lines       19765    19774    +9     
=======================================
+ Hits        19279    19288    +9     
  Misses        268      268           
  Partials      218      218           

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[gmlewis]

Thank you, @kyungseopk1m!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra