Allow using Mercure with other transports than SSE

[dunglas]

Mercury is essentially a pub/sub protocol for resource representations in the sense of REST and HTTP. It allows a client to use the HTTP protocol to receive new versions of one or multiple resources. It also provides an advanced authorization system to ensure that a given client is allowed to receive a given update.

Currently, the spec is tight to the Server-Sent Event protocol. While SSE is a good default and is better than most other solutions for the common use cases, it also has some limitations:

• binary support is limited (binary data need to be base64-encoded)
• while possible, it's not really adapted to send updates to a server

Most of the spec is already transport-independent, and it could be useful for some use cases to allow using the Mercure protocol with other transports than SSE:

• As requested in #100, WebSub could be used to send updates to a server instead of to a client. As Mercure has been largely inspired by WebSub, adding WebSub as a new transport should be easy.
• WebTransport and WebSockets could be proposed as alternative transports for binary data
• Braid's subscribe feature could also be used if it gains traction (even if it would probably be a better idea to use Mercure for the subscribe feature of Braid)

I propose to explicitly state in the spec that Mercure can be used with different transport. To move the parts of the spec related to SSE in a dedicated "SSE transport" section and to a new "WebSub" transport section. WebSockets, WebTransports, and Braid transports could be defined as other specs later if asked by the community.

[gedw99]

Websub is something i have used before. Would be pretty nice for it to be part of Mercure.

@dunglas Do you know if there is any code to look at yet to hack on in a branch ?