[wsrp] Extension namespaces and schemas

Subbu and I have done a bit of off-line
interaction on this question and the suggestion we propose is:

For those extensions with a high expectation
that they will be stable once approved, the wsrp-extra namespace/schema
should be used. Note that only additive changes to the schema for this
namespace on the docs.oasis-open.org server should be allowed.

For those extensions lacking this expectation
of stability, unique namespaces/schemas should be used.

Rich