Expose protocol version

[lann]

e.g.:

variant version {
  HTTP11,
  HTTP20,
  HTTP30,
  other(string),
}

incoming-request-version: func(request: incoming-request) -> version

It would be nice to make the labels e.g. HTTP-1-1, but this isn't currently allowed; see https://github.com/WebAssembly/component-model/issues/134