Add array_average function

Would be great if trino can add array_average and array_sum functions like in presto

array_average(array(double)) → double#
Returns the average of all non-null elements of the array. If there is no non-null elements, returns null.

array_sum(array(T)) → bigint/double#
Returns the sum of all non-null elements of the array. If there is no non-null elements, returns 0. The behavior is similar to aggregation function sum().

T must be coercible to double. Returns bigint if T is coercible to bigint. Otherwise, returns double.

https://prestodb.io/docs/current/functions/array.html

[mdesmet]

From an end user perspective, it sill makes sense to add these functions similar to other database engines: It will be much easier to use and much readable and compact using array_average compared to subqueries or UNNEST.

One question i had in the PR is about rewriting those queries into some construct already supported (array_reduce/unnest/…) or a proper implementation.

@martint, @ebyhr : Can we make a decision, if we want to support this function? if yes how to support it by rewriting or by a proper implementation.

IMHO rewriting is better, but i would rather rewrite it to array_reduce then to unnest.

[ebyhr]

Let me change the title as there is another issue for array_sum in #4346