Avoid creating a new HttpClient on every request

[whitfin]

Seems unnecessary to keep creating one, as we can just keep the client instance in the main GroupsIOApiClient and allow it to be closed. I'll gladly contribute this back if you're willing to accept the changes :)

[nightowlengineer]

Yep, happy for any/all PRs on this project. Haven't had much time recently to finish the new/updated calls off, and so other things like a proper review of things like that haven't taken place yet. If you're happy to do the work that would be great :-)