Add basic health checks

[davissp14]

Add some basic health checks to cover:

1. Connectivity
   Something that periodically issues a PING would be fine.

2. Connections
   Something that communicates active connections/max connections.

3. Disk/Memory/CPU/IO
   Same VM stats as PG would be fine. However, we will need to tweak memory alerting a bit for configurations with a memory eviction policy set.

There's plenty more we could add, but I think this would be a good start.

[davissp14]

We should leverage the check library that we use for postgres-ha/postgres-standalone:
https://github.com/fly-apps/postgres-ha/tree/main/pkg