propagate encoded_plan usage from REST API to N1QL

Taking inspiration from https://forums.couchbase.com/t/execute-encoded-plan-through-n1ql/8407

It would be good to allow using encoded_plan in n1ql, for example

PREPARE "<string containing an encoded plan>" // decodes encoded plan and creates new prepared statement

EXPLAIN "<encoded plan>" // decodes and explains

or even having a function PLAN_EXPLAIN(plan), to be used in conjunction with the monitoring system keyspaces, e.g.

SELECT PLAN_EXPLAIN(encoded_plan) FROM system:prepareds
WHERE name = "....."

which would explain a specific prepared statement.