HTTP2 - 100 continue triggered by content request

[danielkec]

• Investigate if Allow Helidon server to properly implement Expect: 100-continue protocol #4912 functionality is feasible with HTTP/2 - it is

https://httpwg.org/specs/rfc9113.html#rfc.section.8.8.5

  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

• Client part 4.x: HTTP/2 Client 100 continue #7604
• 4.x: HTTP/2 Server 100 continue #7632