support count aggregation function

We already have a simple aggregation implementation in V1 https://github.com/Fedomn/sqlrs/blob/main/src/executor/aggregate/simple_agg.rs, that could be a reference.
A different point is that this function need migrate into V2 planner, that could refer to https://github.com/Fedomn/sqlrs/blob/main/src/planner_v2/operator/logical_limit.rs and https://github.com/duckdb/duckdb/blob/main/src/planner/operator/logical_aggregate.cpp