Add a new non-clustered index on the sta_pr_participant table to optimize queries that look up pull request participants by role and user ID.

The aim is to improve response times of the /rest/api/latest/projects/{projectKey}/repos/{repositorySlug}/participants REST endpoint for repos that have a high number of rows in the sta_pr_participant table.

The improvement request is specific to instances that use MS SQL Server, where the current indexes on the sta_pr_participant table may not adequately cover the query patterns used by the endpoint. Other supported databases may handle these queries differently due to differences in query planning and index utilization.