## Is your feature request related to a problem?

You can currently install plugins from Github and NPM sources, but not GitLab

## Describe the solution you'd like

Copy a GitLab URL to install a plugin

## Describe alternatives you've considered

Using the plugin source editor.

## Additional context

Came from a conversation in the Users' slack

#### *Thank you* for your feature request – we love each and every one!

posthog/plugins/test/mock.py
import base64
import json
from .plugin_archives import (
    HELLO_WORLD_PLUGIN_GITHUB_ATTACHMENT_ZIP,
    HELLO_WORLD_PLUGIN_GITHUB_ZIP,
    HELLO_WORLD_PLUGIN_NPM_TGZ,
)
def mocked_plugin_requests_get(*args, **kwargs):
    class MockJSONResponse:
        def __init__(self, json_data, status_code):
            self.json_data = json_data
            self.status_code = status_code
        def json(self):
            return self.json_data
        def ok(self):
            return self.status_code < 300
    class MockTextResponse: