DO NOT MERGE Initial rabbit_web work

[lhoguin]

Proposed Changes

The goals of this PR are multiple:

• Do a good refactoring of how we start and manage listeners; starting with only Web listeners
• Add proper Websocket over HTTP/2 support
• Add HTTP/3 support (including Websocket over HTTP/3, though browsers aren't ready yet I believe); this involves automatically producing an Alt-Svc header to allow browsers to find the QUIC endpoints
• Make it possible for Web applications to share a listener (simplified environment / configuration, simplified example plug-ins)
• Provide a path to fix a long standing issue in dual stack (IPv4/IPv6) environments where it's not possible to find the correct listener in some conditions

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)

[lin72h]

This is very exciting new feature, thanks cowboy!