Check directory length in pre-flight check for Postgres socket

What does this Merge Request do and why?

If GDK is installed into a directory with a name that is too long, Postgres will fail to start if it's configured to use a Unix socket, as Unix socket paths have fairly small maximum lengths. A deeper technical explanation as to why can be found in the Postgres commit that added the check.

This MR adds a pre-flight check that attempts to create a socket of equivalent length to the Postgres one and fails the pre-flight check if the socket cannot be created due to a path length issue. The check is bypassed if GDK is configured to use TCP.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

/cc @ashmckenzie

Closes #1481 (closed)