feat(gitlab): Add API pipleine flag

[evanpurkhiser]

Part of VDY-39: GitLab: API-driven integration setup

[linear-code]

VDY-39 GitLab: API-driven integration setup

[sentry]

Bug: The new backend feature flag organizations:integration-api-pipeline-gitlab is exposed to the API but lacks the required frontend implementation, rendering it non-functional.
_{Severity: MEDIUM}

Suggested Fix

Either remove the organizations:integration-api-pipeline-gitlab feature flag until the frontend implementation is ready, or add the required frontend code. This includes adding a 'gitlab' entry to the API_PIPELINE_FEATURE_FLAGS map and registering a gitlabIntegrationPipeline in the frontend pipeline registry.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: src/sentry/features/temporary.py#L146

Potential issue: The new feature flag `organizations:integration-api-pipeline-gitlab` is
registered in the backend with `api_expose=True`, indicating it should be used by the
frontend. However, the corresponding frontend implementation is missing. The
`API_PIPELINE_FEATURE_FLAGS` map in
`static/app/views/settings/organizationIntegrations/addIntegration.tsx` lacks an entry
for 'gitlab'. This causes the `getApiPipelineProvider` function to return `null`,
forcing a fallback to the legacy popup-based integration flow. As a result, enabling the
feature flag has no effect on GitLab integration setup.

_{Did we get this right? 👍 / 👎 to inform future reviews.}