Fix N+1 problem in the pipeline bridges API endpoint

What does this MR do and why?

The "GET /api/:version/projects/:id/pipelines/:pipeline_id/bridges" endpoint renders the user relation of the bridges. Besides, followers and following are rendered for each user, so all of these raise N+1 SQL query problems.

In this change, we are adding "user: [:followers, :followees]" to preload these.

Related to #435069 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Current duration, DB count, and DB duration of this endpoint; https://log.gprd.gitlab.net/app/r/s/lCCkV

I expect that this will get lower.