protoc-gen-openapi and export improvements.

This contains several improvements to protoc-gen-openapi from @jeffsawatzky , including the ability to use proto annotations to add fragments to generated OpenAPI documents (#324). It also adds some missing OpenAPI compiler code that Jeff uncovered when testing the new fragment overlay feature - maps of strings were not properly exported in ToRawInfo() methods in both OpenAPI packages (v2 and v3). Thanks, Jeff!