Use @TypeParameter to simplify multimap_agg aggregation function

[ZacBlanco]

With the merge of #21316 we can now simplify this aggregation function implementation by using @TypeParameter annotations in the serializer and state factory constructors, which means the function can be entirely converted to using the function annotation framework. This will simplify the implementation and improve maintainability.

[weimingdiit]

@ZacBlanco i would like to work on this.