Get rid of plain text credentials in push mirroring config (align with pull mirroring)

Currently when setting up Push to a remote repository there's no way around configuring plain text credentials. Even the documentation[0] suggests putting a plaintext token in the config......

I assume I don't need to explain why this is a security issue.

There should be a solution like was implemented for the Pull from a remote repository (the config is near identical, why was it only done for pull mirroring?) where a password field appears when password authentication is required.

There's already an issue for ssh mirroring support[1], but this is about configuring credentials, not support for any particular method.

[0] https://docs.gitlab.com/ee/workflow/repository_mirroring.html#setting-up-a-mirror-from-gitlab-to-github
[1] https://gitlab.com/gitlab-org/gitlab-ee/issues/3251