Major Refactor! Migrate to pb33f/libopenapi

[sudorandom]

Resolves #12 and #11.

This refactor switches out github.com/swaggest/jsonschema-go and github.com/swaggest/openapi-go for github.com/pb33f/libopenapi to get access to several features: ordered items and more complete support for the 3.1 spec.

A couple things changed with the output:

• The order of everything is all switches up but from now on paths and schema properties will appear in the same order as they are defined in the protobuf file as methods and fields.
• components.requestBody is no longer used. The bodies are simply defined in This simplified a lot of things by not having an extra point of indirection
• google.protobuf.Empty is now closer to the actual behavior in JSON (an empty object, {})
• Adds more support for gnostic annotations, notably all the components tree
• Adds support for:
  • google.protobuf.NullValue
  • google.protobuf.StringValue
  • google.protobuf.BytesValue
  • google.protobuf.BoolValue
  • google.protobuf.DoubleValue
  • google.protobuf.Int64Value
  • google.protobuf.Uint64Value
  • google.protobuf.FloatValue
  • google.protobuf.Int32Value