Add config param all to enable syncing all projects in GitLab instance

[brendan-kellam]

This PR adds the option all to the GitLab index config. When enabled, all projects within the GitLab instance that are visible to the provided token (if any) will be indexed.

Example:

{
    "$schema": "./schemas/v2/index.json",
    "repos": [
        {
            "type": "gitlab",
            "url": "https://gitlab.example.com",
            "token": {
                "env": "GITLAB_TOKEN"
            },
            "all": true
        }
    ]
}

Note: I've made this option no-op when using https://gitlab.com . I tested it, and if all is enabled, all projects hosted on gitlab.com are fetched, resulting in the get request never completing. We could use the visibility param of the GET /projects api to limit things to just private or internal (limiting things to only the projects the token can see), but I will leave that as a future improvement if there are requests for it.

Fixes #49