Support search by "source branch" on MR list page

What does this MR do and why?

Tier: GitLab Free

Environment: ~SaaS self-managed

Related issue: #213177 (closed)

Related Jihu Epic: https://jihulab.com/groups/gitlab-cn/-/epics/5470

Support search by source branch on the MR list page.

SQL changes involved

Every time we use the source branch query on the UI, these two SQL will be executed:

SELECT
 COUNT(*) AS "count_all",
 "merge_requests"."state_id" AS "merge_requests_state_id"
FROM
 "merge_requests"
WHERE
 "merge_requests"."target_project_id" = 6
 AND "merge_requests"."source_branch" = 'dev'
GROUP BY
 "merge_requests"."state_id"


SELECT
 "merge_requests".*
FROM
 "merge_requests"
WHERE
 "merge_requests"."target_project_id" = 6
 AND ("merge_requests"."state_id" IN (1))
 AND "merge_requests"."source_branch" = 'dev'
ORDER BY
 "merge_requests"."created_at" DESC,
 "merge_requests"."id" DESC
LIMIT
 20 OFFSET 0

Screenshots or screen recordings

How to set up and validate locally

Just switch to the current branch support-source-branch-filter and enter the MR list page of any Project or Group.

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.

cc @prajnamas @mtan-gitlab @icbd