Add ability to retrieve query plan via REST api

The ability to retrieve the value of a call to org.modeshape.jcr.api.query.QueryResult.getPlan() for any given JCR query via the REST api would be very useful.

Another possible approach (or additional enhancement) would be to support this as an extension in JCR-SQL2 in the common form of the EXPLAIN keyword, ie:

EXPLAIN SELECT e.* FROM [inf:episode] AS e INNER JOIN [inf:patient] AS p ON ISDESCENDANTNODE(e,p) WHERE p.[inf:masterId] = '00010014'

This second approach should also make access to the query plan possible via the JDBC driver.