This CL is an alternative to PR #957.

Notable differences:

- This does not use the regular proto generation infrastructure
as the idea is to combine more than just Protobufs  as a source
for the OpenAPI schema (e.g. Go code and CUE files)

- All files are self-contained, but not do repeat imported types:
types are included as a component with a fully qualified path.

- Fixes several issues relative to #957:
  - uses JSON names
  - fixes oneOf (current representation reflects the proto accurately)
  - accurate interpretation of protobuf types, even if not
    supported by OpenAPI (e.g. uint64)

Current issues:

- Proto types with standard mappings to JSON still map
  to the proto type. Question: support both or just JSON mapping.
- Not printing document info and version yet.

- files are now generated according to build plan: envoy files
are removed, networking files are split
- corrected typo
- comments shortened