Allow cli extensions to be installed

Problem to solve

currently there is no mechanism in place for glab to allow extensions to be added/installed. This feature will encourage users to contribute to the CLI, while adding new features for users to interact with their glab instance without recreating the login/auth flow.

Proposal

Github's gh cli allows for extensions to be installed as shown here: https://cli.github.com/manual/gh_extension It will require some additional logic to pass down commands from glab to the extension. Its very easily replicate-able by walking through gh's open source code on github.

Further details

I built the gitlab buddy chrome/firefox extension, and was looking into the gh dash extension, and would love to implement something similar for gitlab.

Links / references

https://github.com/cli/cli/blob/trunk/cmd/gh/main.go#L107