PgBouncer: use service FQDN

[guidoiaquinti]

Description

We currently do not use a FQDN record to connect to pgbouncer. Due to Kubernetes internals, this will translate in several DNS requests before receiving the right answer:

bash-5.1$ nslookup posthog-pgbouncer
Server:		172.20.0.10
Address:	172.20.0.10:53

Name:	posthog-pgbouncer.posthog.svc.cluster.local
Address: 172.20.88.150

** server can't find posthog-pgbouncer.cluster.local: NXDOMAIN

** server can't find posthog-pgbouncer.cluster.local: NXDOMAIN

** server can't find posthog-pgbouncer.svc.cluster.local: NXDOMAIN

** server can't find posthog-pgbouncer.svc.cluster.local: NXDOMAIN

** server can't find posthog-pgbouncer.ec2.internal: NXDOMAIN

** server can't find posthog-pgbouncer.ec2.internal: NXDOMAIN

This PR adds a new posthog.pgbouncer.fqdn definition in our templates, that can be used to directly target the FQDN record.

Related to #628

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

CI is ✅

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works