Full support for custom provider aggregate functions

[roji]

Following discussion in #13278, I've done some exploratory work in npgsql/efcore.pg#1531 on PostgreSQL aggregate functions. Here are some issues I ran into:

• It seems impossible to add support for aggregate functions in GroupBy - GroupingElementExpression is private, etc.
• For top-level (IQueryable) invocations, lots of code needs to be duplicated from relational (see https://github.com/npgsql/efcore.pg/pull/1531/files#r502918288).
  • Weak entities in particular seem impossible to support (https://github.com/npgsql/efcore.pg/pull/1531/files#r502918905)

/cc @smitpatel

Interesting candidate translations:

• GROUP_CONCAT for MySQL (see #26838), also see other MySQL functions on that page. In PostgreSQL this is called string_agg.
• json_agg/jsonb_agg on PostgreSQL, JSON_OBJECTAGG on MySQL.
• array_agg on PostgreSQL.

[smitpatel]

GroupBy aggregate issue is not related to custom provider function. Not all GroupBy patterns work right now and no one can extend them either that is one of the part in #18923

For second point, just need to make TranslateExpression protected. But that issue is not related to aggregate either. Any custom queryable method will suffer that.

[roji]

OK, thanks - it would be nice if we could make all this work in 6.0. It seems useful to me to track here everything what's needed for custom provider aggregate functions, but up to you if you feel everything's covered elsewhere.