REST API: add an endpoint to configure pull mirrors
### Problem
Currently, pull mirroring configuration [is done through Projects API](https://docs.gitlab.com/ee/api/projects.html#configure-pull-mirroring-for-a-project). This is confusing and leads to various problems.
### Customer impact
It's impossible to provide username or password for Pull mirror via existing API if they contain `/` character, for example.
See details: https://gitlab.com/gitlab-org/gitlab/-/issues/465311#note_2128504641
> URL-encoding of `/` to `%2F` did not work as it is never decoded before being sent to the remote repository for authentication. When doing so, we get the following error message: `{ "message": "http://<REDACTED>/git/myrepo.git/ endpoint error: 401 Unauthorized" }`
[source](https://gitlab.com/gitlab-org/gitlab/-/issues/465311#note_2128878857)
### Proposal
1. Create a new endpoint to configure Pull Mirroring for a project. Similarly to [Remote Mirrors API](https://docs.gitlab.com/ee/api/remote_mirrors.html#update-a-remote-mirrors-attributes).
2. Deprecate pull mirror configuration via Projects API.
issue
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