## 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
if args[0] == "https://github.com/PostHog/helloworldplugin/archive/{}.zip".format(
        HELLO_WORLD_PLUGIN_GITHUB_ATTACHMENT_ZIP[0]
    ):
        return MockBase64Response(HELLO_WORLD_PLUGIN_GITHUB_ATTACHMENT_ZIP[1], 200)
    if args[0] == "https://registry.npmjs.org/posthog-helloworld-plugin/-/posthog-helloworld-plugin-0.0.0.tgz":
        return MockBase64Response(HELLO_WORLD_PLUGIN_NPM_TGZ[1], 200)
    if args[0] == "https://raw.githubusercontent.com/PostHog/plugin-repository/main/repository.json":
        return MockTextResponse(
            json.dumps(
                [
                    {
                        "name": "posthog-currency-normalization-plugin",
                        "url": "https://github.com/posthog/posthog-currency-normalization-plugin",
                        "description": "Normalise monerary values into a base currency",
                        "verified": False,
                        "maintainer": "official",
                    },
                    {
                        "name": "helloworldplugin",
                        "url": "https://github.com/posthog/helloworldplugin",
                        "description": "Greet the World and Foo a Bar",
                        "verified": True,
                        "maintainer": "community",
                    },
                ]
            ),
            200,
        )
    return MockJSONResponse(None, 404)