JDBC-based cache stores require a DataSource attribute for runtime

Currently, there are 2 attributes for defining the DataSource of a JDBC-based cache store:
1. "data-store", which references a named DataSource capability
2. "datasource", the legacy attribute which specifies the jndi name of a DataSource

Both attributes are currently nullable. Consequently, the model does not require either to be specified, which would cause the runtime to fail.
The data-source attribute should be required, with "datasource" set as an alternative.

No transformer is necessary since "datasource" was a required attribute in older versions of the model.