Add support for repository access tokens when using BitBucket

[MarthaScheffler]

Feature description

At the moment, the Git plugin only allows authentication as a user, with either password or personal access token.

It would be great to add an option to use repository access tokens. so that we can e.g. pull the code from the dbt repo for a production run using a repository access token, and not a personal one.

[nicoallgood]

For context: Repository Access Tokens documentation (which as far as I can tell, seems to be a Bitbucket-specific concept)