Audit CI runner event when assigning to project

Audit need

When admins, group/project owners/maintainers assign a runner to a project, it is important to preserve a trail of data of the event.

Proposal

Add the following audit events for when a GitLab Runner is assigned to a project.

• Date and timestamp in UTC
• Public IP address (i.e. the source IP address from where the registration command was run)
• Runner short SHA
• Newly assigned project

Implementation

• 1. Move the logic to assign a runner to a project from API::Ci::Runners to a new AssignRunnerService class.
• 2. Move the logic to unassign a runner from a project from API::Ci::Runners to a new UnassignRunnerService class.
• 3. Add auditing in AssignRunnerService#execute method.
• 4. Add auditing in UnassignRunnerService#execute method (!81540 (merged)).