[CBAS] Add MEDIAN() and ARRAY_MEDIAN() Functions

MEDIAN() is supported as a built-in function in Analytics from Couchbase Server 7.2.5.

• The MEDIAN function is an Aggregate Pseudo-Function. If this function is used in a query without an explicit GROUP BY clause, it implicitly forms all results into a single group.
• There is an ARRAY_MEDIAN function for explicit use with arrays, but STRICT_MEDIAN has not been implemented.
• This function supports the FILTER clause.
• This function does not support the DISTINCT keyword, unlike other aggregate functions.

This has been implemented in AsterixDB (neo branch). Current plan is to add the documentation in AsterixDB, and let the Antora pull the changes into the Analytics documentation. Will need to add a note to the Analytics docs to say that MEDIAN and ARRAY_MEDIAN are only supported in 7.2.5 / 7.6.2 and later.

Need to update Queries › Grouping › Aggregation Pseudo-Functions. Fix the order of functions in the Aggregate function table while we're at it.