Reject unknown fields in configuration files

[divergentdave]

This changes all config-related deserialization routines to throw an error upon seeing unexpected fields, rather than ignoring them. This should be mentioned in the next release notes.

[branlwyd]

I'd like it if we could apply this to every serde-serialized structure by default, but I can't find a way to do this.

[divergentdave]

That would be possible if we rolled up our sleeves to write some proc macros, and then used our derive instead of serde's derive everywhere, but I don't know if that would be worth it.

Moreover, I am intentionally holding off on changing deserialization done on aggregator API requests for now. We may want to make that parsing strict as well at some point, but it will take a whole different set of testing, and affect a different set of API consumers.

[branlwyd]

I definitely don't think writing proc macros ourselves is the right path. If we really wanted to address this, I'd try to upstream a feature into serde before doing that -- but I don't think that is worth it, either. :)