REST API: add an endpoint to configure pull mirrors

Problem

Currently, pull mirroring configuration is done through Projects API. 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: #465311 (comment 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

Proposal

1. Create a new endpoint to configure Pull Mirroring for a project. Similarly to Remote Mirrors API.

2. Deprecate pull mirror configuration via Projects API.