Support custom OmniAuth strategies in Omnibus packages

Description

To configure a custom OmniAuth strategy we need to add the gem to the Gemfile and install it. This is currently not possible if GitLab EE is installed using the Omnibus packages.

Proposal

There should be a configuration option to include and install gems for additional OmniAuth strategies.

Links / references

https://docs.gitlab.com/ce/integration/omniauth.html#using-custom-omniauth-providers