Add a gRPC Web to HTTP/2 codec (#1047)

Motivation:

Following from #1041, we'd like to switch our currency type on the
server from HTTP/1 to HTTP/2. This means that to retain support for gRPC
Web we need a codec to convert between HTTP/1 and HTTP/2 in the opposite
direction to the codecs provided by NIO HTTP/2.

Modifications:

- Add a gRPC Web to HTTP2 codec

Result:

- Nothing yet, as this remains unused.

Author: glbrntt