[native_assets_cli] Use a schema and schema checker for hook config and output

[dcharkes]

We should consider

1. generating a an implementation for the build/link config and build/link output rather than handwriting it
2. use some kind of schema spec to generate from and check backwards and forwards compatibility (due to version skew between the Dart SDK and hook helper packages).

The schema evolution checks will validate we don't introduce any syntactic errors.
The validators for different parts of the config / output validate semantic errors.

Since we don't validate syntactic errors, we should not do any breaking changes to syntax (only when we can bump an SDK constraint). For example, nullable fields in the schema should always stay nullable, even if semantically they are guaranteed to not be null under certain conditions. See the discussion on #1824, thanks @mkustermann!

We can introduce a schema definition, a schema evolution checker, and a code generator from that schema. Then we can make the semantic API (the user API) take the syntactic classes as input (rather than the JSON), and ditto for the validator functions.

Related:

• [native_assets_cli] Document the protocol #95