Support for multiple GitLab instances - e.g. gitlab.com and self-hosted

[Damian Nowak]

Here's an example how this issue was resolved in a different plugin - Gitlab MR:

"gitlab-mr.accessToken": "ACCESS_TOKEN_FOR_GITLAB.COM",
"gitlab-mr.accessTokens": {
    "https://gitlab.domain.com": "ACCESS_TOKEN_FOR_GITLAB.DOMAIN.COM"
}

Which instance to use should be inferred from the git repo URL.