postgres-util: refactor implementation to use openssh instead

[andrioni]

Refactor implementation of SSH tunnels to use openssh instead of async-ssh2-lite due to the issues we've been facing so far.

I'll add tests here soon, I just wanted to get more eyes on it soon, because jetlag.

Motivation

• This PR fixes a recognized bug: #14566

Checklist

• [ ] This PR has adequate test coverage / QA involvement has been duly considered.

• [ ] This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-protobuf label.

• [ ] This PR includes the following user-facing behavior changes:

[andrioni]

FYI, I should push the fixes for the comments as soon as I fix the mzcompose test failure: it's something to do with the permissions when openssh reads the key in the container. It's probably something obvious that I'm forgetting, it just takes a while because I have to build a clean docker image to test things.

[guswynn]

@andrioni sounds like a great plan!

[andrioni]

OK, the issue was completely different, we just didn't have openssh installed in the container 🤦.

Added tests for Postgres with SSL, and merging.

[guswynn]

🎉