Update messaging spec

[JohanMabille]

This adds the missing specification for the accepted JEPS of these last years, in order to make a new release of the specification. I'll make a separate PR for the handshaking and the regular connection patterns.

[minrk]

Great, thank you!

[rgbkrk]

For previous client bindings that used PUB/SUB sockets for IOPub, will they be backwards compatible or do they need to be able to detect that the version is 5.5 somehow?

Based on reading Impact on Existing Implementations, it seems like I should add this into the jupyter rust crates (jupyter-protocol, runtimed) and get it into Deno and Zed.

For now I'll add the message in to the Rust crate in runtimed/runtimed#203 and come up with a plan for how I can support XPUB kernels maybe optionally at first (?).

[minrk]

If a kernel speaks protocol version 5.4 and uses a PUB socket, the iopub_welcome message will be missing when subscribers connect. Clients expecting protocol version 5.5 should understand this and not wait for this message. It is otherwise indistinguishable from the client side. If a client doesn't wait for iopub_welcome, there is no difference.

[minrk]

Suggested change

	The **IOPub** PUB socket is replaced with an XPUB socket.
	The **IOPub** PUB socket is replaced with an XPUB socket,
	to enable the `iopub_welcome` message.
	There is no other difference between kernel PUB and XPUB sockets from the client perspective.