Add EffectfulQueryBuilder for higher-level API on top of RootEffect / GenericMapping

[dantb]

Initial rough attempt at a higher-level library API combining RootEffect and GenericMapping to build a read-only GraphQL API. Trying to add some type-safety to the arguments and hide away select elaboration, RootEffect, genericCursor etc, behind a builder pattern. Managed to allow constructing EffectfulQuery values independently and compose them; eventually calling build with a schema to create the mapping. Ideally the schema / queries would be tied together at compile time, but I imagine that would be wholly invasive and quite complicated... The Caliban library seems to do that, though it didn't work as expected when I last tried it.

Still needs a lot of tidy up, particularly to make adding arguments better. Perhaps another builder pattern in the construction of an EffectfulQuery, to avoid the verbose arg1, arg2 constructors.

Note to self - this is currently blocked by there being no way to create a CursorBuilder other than using semiauto which is in the mapping scope. Ask for help on this - either that can be extracted or we can add options for manual creation of CursorBuilders, similar to circe Encoders.