Refurbish indexerconfig

[ala-mode]

fixes #500 also #506 relevant.

WIP.

[zancas]

Hey! I really like the way this is looking.. anything that simplifies zaino-testutils is probably very worthwhile.

[ala-mode]

@pacu or anyone else; this is currently a WIP.

One architecture decision (to put a gRPC related module in zaino-common) was my own attempt to fit things in, and I was not sure of.

Discussions around specifics (and a running list of config fields) can be seen at issue #500

[ala-mode]

@nachog00 mentioned 'stale pr' #467 on issue 500. That one was started easrly August, and last commit was August 25. It's about +10k lines -3k lines.

Mostly because of the size of the stale PR, I'm probably only going to use it for reference if I find myself stuck.