This commit implements RFC 9218 extensible prioritization scheme. It
is enabled when a local endpoint submits
SETTINGS_NO_RFC7540_PRIORITIES = 1. This commit only handles priority
signal in HTTP request header field. Priority header field in
PUSH_PROMISE is not supported.
HTTP messaging must be enabled to take advantage of this
prioritization scheme because HTTP fields are not parsed if HTTP
messaging is disabled.
Tatsuhiro Tsujikawa