Make a limit for Gitlab Shell operations configurable

Problem to solve

Currently, we have gitlab_shell_operation rate limiter that is set to 600 requests for action/project/user combination per minute.

Customers may want to specify their own values:

• Increase it to allow more git operations being performed
• Decrease it to prevent from abusing this endpoint to improve scalability

Potential solutions

One of the recent examples of implementing a configurable rate limit is Add limits for Projects List API for unauthenti... (!112283 - merged)

Implementation

• Add gitlab_shell_operation_limit to application... (!123456 - merged)
• Allow `gitlab_shell_operation_limit` setting to... (!123761 - merged)