feat: add check to verify backend connection in readiness probe

[nancynh]

This PR adds an optional flag to set that will continuously check that a connection can be established to the server before returning a ready status in the health probe.

Fixes #909.

[enocom]

We can skip the for loop as Cloud Run (or K8s) will continue to invoke this endpoint at a regular interval. See https://docs.cloud.google.com/run/docs/configuring/healthchecks#readiness-probes (especially the "period" parameter).

Since these checks will result in prematurely closed database connections, what do you think about extending the metadata exchange to indicate this is a health probe (and so don't need to initiate a connection to the db)?

[nancynh]

Removed the loop.

Fine with possibly integrating this with metadata exchange though that'll need some more thinking of how we go about that first...

[enocom]

Code looks good. I think we should double-check with our Cloud Run friends if we have the correct understanding of Direct VPC Egress setup times.

[nancynh]

Discussed this a bit offline - moved the check to be in the startup probe instead in consideration of https://github.com/learnk8s/kubernetes-production-best-practices/blob/master/application-development.md#apps-are-independent.

Gonna hold off on merging this PR though to see what the Direct VPC Egress folks say first

[enocom]

We might want to manually test this to confirm it works as intended. Otherwise LGTM.