Http2 header parser can be made more performant

[pjfanning]

pekko-http/http-core/src/main/scala/org/apache/pekko/http/impl/engine/http2/RequestParsing.scala

Line 194 in af7ed86

// FIXME: later modify by adding HttpHeaderParser.parseHttp2Header that would use (name, value) pair directly

• currently converts the strings back to bytes to parse
• adds a hacky 'x' to the content to get the current parser to accept the byte stream
• may need a dedicated new parser method that works at char or String level