Add PG connect_timeout parameter

What does this MR do?

This change adds ability to configure the connect_timeout parameter in database.yml (for Rails). This controls the timeout for a connection attempt to the database host. By default, this is unlimited - which is undesirable in case a database host is not reachable at all. In particular, when load balancing is enabled and a replica becomes unavailable, any connection attempt would basically stall the application.

Now this change adds the parameter, but doesn't set a default. This is intentional for now, since we first want to apply this on GitLab.com only. We can ship a default value later.

Related issues

gitlab#238556 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened -> gitlab-org/charts/gitlab#2294 (closed)