Decode/Encode unaware of Selected Sub-Protocol

Although the sub-protocol is negotiated with with the client, there is no way to know what that is as this is only available in `Session#getNegotiatedSubprotocol()` which is not available to `Decoder` or `Encoder`. The use-case is to have subtly different encoding/decoding based on sub-protocol.

Start by reading the Decoder, Encoder, and Session#getNegotiatedSubprotocol() entry points to understand how sub-protocol negotiation is exposed today. Determine how encoding and decoding could access the negotiated value, and define completion as allowing them to vary behavior based on that sub-protocol without breaking existing use.

Written by the indexing model from the issue text.