Generate OpenAPI client for Error Tracking in vendor/gems

What does this MR do and why?

This MR adds a new rake task rake gems:error_tracking_open_api:generate to generate and update OpenAPI client needed for Category:Error Tracking feature.

This client is vendored as a gem in vendor/gems/error_tracking_open_api. Currently, no manual changes are made to the generated code.

Needed for !90122 (merged).

feature flag See Rollout Plan for Error Tracking backed by Click... (gitlab-org/opstrace/opstrace#1728 - closed).

How to set up and validate locally

bundle exec rake gems:error_tracking_open_api:generate

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.