Replace openapiv2 path template parser with httprule parser

[johanbrandhorst]

We have at least two parsers for the http.proto template language;

• In the httprule package, which is used by protoc-gen-grpc-gateway to generate the handler paths
• In the genopenapi package to generate the OpenAPI paths.

We should unify these two places, and we should be using the one in httprule. I did a little bit of investigating and I think it should be possible without too much work, and then we'd have consistency between the two generators.