[Release/3.7] ch20507 add sshBastionServer.customHostname in PrimeHub values

[kentwelcome]

PR checklist

• [x] Ensure you have added or ran the appropriate tests for your PR.
• [x] DCO signed

What type of PR is this?

• If the customHostname is configurated, ssh server will show this hostname in jupyterhub spawner page.

What this PR does / why we need it: Feature used by AWS one click

Which issue(s) this PR fixes: ch20507

Special notes for your reviewer:

Does this PR introduce a user-facing change?: