Support flate/deflate compression for websocket connections as per RFC7692

[simonmittag]

Is your feature request related to a problem? Please describe.
RFC 7692 specifies websocket flate compression on a per message basis as an optional extension to the websocket protocol.
Uncompressed frames are still accepted after successful negotiation.

Note this means an API change internally to gobwas/ws, but it is supported in the project.

Describe the solution you'd like

• Accept this extension for downstream websocket connections during handshake to support clients.
• Attempt this extension for upstream websocket connections during handshake to save bandwidth during upstream transfer.

[simonmittag]

this needs to be prioritised