Mirror with access token

In addition to SSH private keys, support access (group, project and personal) tokens for accessing our own repository via HTTPS. Set the new SELF_GIT_PROTCOL variable to https and provide a valid access token in the MIRROR_PRIVATE_TOKEN variable to enable. The script falls back to PRIVATE_TOKEN, which allows users to define a global token for all types of automations.

As a bonus this MR also includes a new variable MIRROR_REMOTE_BRANCH that allows users to override the remote (source) branch in case the default name is different from local. For example, this repository still uses master, but users may now switch to main in their mirrors.