Multimodule repo: apps are in "cmd". protoc-gen-openapi improvements. protoc-gen-jsonschema.

This renames the former apps directory to cmd and adds a go.mod for each cmd subdirectory. These directories contain demonstrations and various gnostic-related applications, and putting each in a separate module clarifies dependencies and reduces the apparent dependencies of gnostic itself (as listed in the top-level go.mod). Thanks @shenqidebaozi for making this change and @morphar for advising.

This also includes significant improvements to protoc-gen-openapi from @morphar and @tonybase and a new protoc-gen-jsonschema pluigin contributed by @morphar.