Allow passing context from a protocol crate to an user callback

[dequbed]

This one is especially important for a mechanism like EXTERNAL. Callbacks are per-config and have no per-session data making it very annoying to make decisions based on e.g. a provided TLS client certificate.

A possible solution would be to also use the property system here extending the SessionData passed into callbacks already.

I'm also very much open to alternative suggestions!

This is a good first issue since SessionData is only seen by callback and (possibly) the protocol crate and the property system could be used as-is if that ends up being the chose solution.

[dequbed]

As a note for prospective fixers or future me: This issue sadly isn't easy (anymore) since SessionData has been (accidentally) ossified into the public API. This means this issue has to be solved in a breaking release.