misc(proto): add ConfigSettings fields

[paulirish]

fixes #15193

I adapted the suggested syntax quite a bit. But... it's now happy for python, go, and C++ protobuf handling.

with the additions we can delete some of the hax in proto-processors (which is used in LR)

[brendankenny]

Probably good to incentivize stability, but this means we can't change these types later (e.g. string to a struct or whatever), just retire them and pick a new name, right?

Also does it matter if e.g. auditMode/gatherMode are bool in the proto but really boolean | string? string is unusual outside of development of LH itself but it is possible depending on how the person is generating their LHRs (I'm assuming smokerider won't run into this?).