Allow auth http.Client timeout to be configurable

Currently the auth package has a hardcoded timeout of 5s which might not be enough for some setups.

Proposal:

• Introduce a auth-timeout flag to Pages: !687 (merged)
• Add the flag to Omnibus: omnibus-gitlab!7268 (merged)
• Add the flag to Helm charts.: gitlab-org/charts/gitlab!3502 (merged)
• Add the setting to the pages admin docs: gitlab!137845 (merged)