eventchecker: support additional map[KEY]VAL types

[willfindlay]

#260 adds support in the eventchecker for map[string]string but it would be nice to future-proof this by adding support for other map[KEY]VAL types as well. Doing so would require some non-trivial refactoring of the eventchecker codegen but I think it's probably worth doing as a stretch goal.

One thing that would make this a lot easier is moving to Go 1.18 to enable us to use generics. You could imagine, for example generic MapCheck and ListChecker types that could replace what we have now.