xmlstream: remove T: AsXml bound from XmlStream type

While the FromXml bound is necessary for the key implementation of the
XmlStream (the `impl Stream`), the AsXml bound is not as intrinsic: the
Sink implementation can take any AsXml implementation on each separate
send invocation.

By removing this bound, we can add extra logic *only* to the parsing,
without having to worry about effects on the sending side.

Concretely, we'll be able to create a wrapper type for XmppStreamElement
which allows us to capture errors during parsing, without having to
worry about what happens during serialisation.

tokio-xmpp/ChangeLog

@@ -25,6 +25,7 @@ Version NEXT:
       - Update hickory-dns to 0.26 (!671)
       - Drive the XMPP client stream in the background (!631)
       - Add option to split XMPP client into read and write half (!631)
+      - Remove T: AsXml bound from XmlStream (!675)
 
 Version 5.0.0:
 2025-10-28 pep <pep@bouah.net>