Handle 100-continue properly in HTTP/2

[fasterthanlime]

RFC 9113 8.8.5 Gives examples of 100-continue:

  HTTP/1.1 100 Continue            HEADERS
  Extension-Field: bar       ==>     - END_STREAM
                                     + END_HEADERS
                                       :status = 100
                                       extension-field = bar

  HTTP/1.1 200 OK                  HEADERS
  Content-Type: image/jpeg   ==>     - END_STREAM
  Transfer-Encoding: chunked         + END_HEADERS
  Trailer: Foo                         :status = 200
                                       content-type = image/jpeg
  123                                  trailer = Foo
  {binary data}
  0                                DATA
  Foo: bar                           - END_STREAM
                                   {binary data}

                                   HEADERS
                                     + END_STREAM
                                     + END_HEADERS
                                       foo = bar

...mostly as an example of how to translate HTTP/1.1? I suppose that's a question for HTTP/1 => HTTP/2 proxies, which.. doesn't happen that often? I don't know. There's not a fully-featured proxy built on top of fluke right now anyway, only trivial ones, so that's a question for later, but: for now, H2Encoder::write_response has this assert:

        // TODO: don't panic here
        assert!(
            !res.status.is_informational(),
            "http/2 does not support informational responses"
        )

Which is probably wrong.

[fasterthanlime]

That also means in the HTTP/2 client state machine, we shouldn't move on to "expecting body" if we receive an HTTP 1xx response — this may already be the case, I haven't checked, but we should be consistent on both sides.