Add validation to incoming config values

Many of the values from the host page or from a via proxy need validation to produce relatvent warnings for the site owner which may otherwise prevent some features from working. The validated values must support a variety of types including objects and arrays. Our current validation/translation checking process is incomplete and inconsistent.

**Currently**

- All values are nulled if they are not set.
- Some values are translated that come from via because it only outputs strings.
- Some values are just validated, but others are not.


**Specifically we need to**

- [ ] Build a validation module that can take a schema and produce errors from a given data set against that schema
- [ ] Don't set a value to null if its missing, just don't include it. (explore if this can be done)
- [ ] Build a schema and validate values coming into the host page (adder)
- [ ] Build a schema and validate values again coming into the sidebar iframe
- [ ] Design a process for versioning the config so we don't have to support too many different versions of values
- [ ] Translate validated values from via that might need to be move to another type for client processing. e.g. `"3"` to `3`

Relates to https://github.com/hypothesis/client/issues/1968