Support for some new aggregation features

[aturki]

Q	A
Type	feature
BC Break	no
Fixed issues	#2063 (Partially)

Summary

Provides implementation of some aggregation operators

[alcaeus]

Thank you for adding those operators. I've left some comments, but have not yet reviewed the tests.

I'll also note that methods in the class are currently sorted alphabetically - I'd appreciate it if you could keep the ordering and move newly added methods to the correct place.

[alcaeus]

I've extracted an interface from the Expr class to ensure that new methods are added to the abstract Operator stage. In addition, I've removed the __call magic. The idea behind the egg builder is to provide an expressive interface to help people along, and the call magic does the opposite. It was a fallback for when we couldn't have methods named switch, default, etc.

[IonBazan]

Thanks @aturki! 🚀