http2: handle 100-continue flow & writeContinue

Adds an implementation for writeContinue based on the h2 spec & the
existing http implementation. ClientHttp2Stream now also emits
a continue event when it receives headers with :status 100.
Includes two test cases for default server continue behaviour and
for the exposed checkContinue listener.

PR-URL: https://github.com/nodejs/node/pull/15039


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>