New method XAJMSContext#createXAJMSContext

A new method is needed on XAJMSContext which creates a new JMSXAContext using the same connection as the existing XAJMSContext and creating a new session.

```
XAJMSContext createXAContext()
```

This is analogous to the existing method createContext on JMSContext, but for XA-capable JMSContext objects.

This method was omitted from JMS 2.0 on the grounds that the JMS specification did not permit the creation of multiple sessions from the same connection in the Java EE web or EJB container. However this is not the right place to enforce that restriction, since it is a restriction on the application, not the application server or resource adapter, and the XAJMSContext interface is for use by the application server or resource adapter, not by applications. Omitting this method places an unnecessary restriction on the ability for application servers to use multiple sessions on the same connection whilst preventing applications doing the same.
#### Affected Versions
[2.0]

Start by inspecting the XAJMSContext interface and comparing it with JMSContext#createContext. Trace the existing XA context creation path and any related API checks. Done means the XAJMSContext API exposes a method that creates a JMSXAContext with the same connection and a new session, with the intended method name resolved.

Written by the indexing model from the issue text.