Healthchecks and autorestarts for computes

[Omrigan]

Problem description / Motivation

Branched off from https://github.com/neondatabase/cloud/issues/14114

At this moment, we can only rely on k8s's signal for compute unavailability, specifically, container process monitoring.

We would like to have an end-to-end healthcheck, which would allow us to detect problems, such as:

1. Postgres does not accept connections
2. compute_ctl crashlooping
3. Network partitioning

Feature idea(s) / DoD

We have a healthcheck mechanism, allowing us to detect compute issues within <30s, and taking appropriate actions, such as restarting.

Implementation ideas

We should have a piece of code inside vm which would respond to a healthcheck.

[stradig]

Not sure we will need that or if Kubernetes is good enough. Putting in the backlog for now.