Typescript generics for main Client methods

[twoojoo]

When working with typescript I find very comfortable to declare the type of the requested data with generics instead of using the "as" keyword.
So I just added generics to the main methods (execute, eachRow and batch) and to the types on which they are based (Row and ResultSet). I skipped graph methods and their types because I'm not really aware of how how they works in Cassandra.

Since every generic has "any" as default type, backwards compatibility should be ok (except for Row switching from being an interface to being a type, which I'm not sure if it could lead to problems in that sense).

Offtopic: it would be nice to set a string union type for hints. I'm only doubting whether hints can be used for user defined types too, which would probably make this useless.

[absurdfarce]

Greetings @twoojoo and thanks for the contribution! Apologies for taking so long to get back to you.

Have you signed the Contributor License Agreement for contributions to DataStax open source projects? If not you can find it at https://cla.datastax.com/. Thanks!