Type array_agg()

Type array_agg()

The return type of `array_agg()` is declared as a `Sequence[T]` where `T` is bound to the type of input argument.

This is implemented by making `array_agg()` inheriting from `ReturnTypeFromArgs` which provides appropriate overloads of `__init__()` to support this.

This usage of ReturnTypeFromArgs is a bit different from previous ones as the return type of the function is not exactly the same as that of its arguments, but a "collection" (a generic, namely a Sequence here) of the argument types.  Accordingly, we adjust the code of `tools/generate_sql_functions.py` to retrieve the "collection" type from 'fn_class' annotation and generate expected return type.

Also add a couple of hand-written typing tests for PostgreSQL.

Related to #6810

Closes: #12461
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12461
Pull-request-sha: ba27cbb8639dcd35127ab6a2928b7b5b3667e287

Change-Id: I3fd538cc7092a0492c26970f0b825bf70ddb66cd