Unable to use a custom v3 forge

[jhaals]

Hi!

There's currently no support for downloading modules from a custom v3 forge. All requests are made using the v1 API due to https://github.com/rodjek/librarian-puppet/blob/master/lib/librarian/puppet/source/forge.rb#L153

It would be awesome if librarian-puppet could support v3 forges that's not puppetlabs.com

I have a local patch where you can specify librarian-puppet install --use-v3-api
Line 153 in forge.rb would then be if uri.hostname =~ /\.puppetlabs\.com$/ || environment.use_v3_api

I just want to verify if that's a good approach before sending the PR.