Add GitLab Groups to list of supported registry providers

[tecandrew]

Issue Type: Feature Request

When attempting to log into Gitlab I am prompted to enter my username and password (token) with no choice to connect to a specific group/registry within GitLab.

Would be great if I were given a choice to connect when GitLab group to connect to (to see only those specific docker containers).

Extension version: 1.1.0
VS Code version: Code 1.45.0 (d69a79b73808559a91206d73d7717ff5f798f23c, 2020-05-07T16:10:24.511Z)
OS version: Linux x64 5.4.0-29-generic snap

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This possibly could be tied to #1968. From my experience, GitLab structures "users" and "groups" a bit oddly when it comes to logins and authorizations.

Gitlab Workflow extension mandates the explicit URL of the domain to connect to (https://gitlab.com/<username> to use user registries/repos/pipelines) when attempting to log in. If https://gitlab.com/ is used when logging in nothing connects (I'm assuming a 401 error).

[bwateratmsft]

Moving to #869