Pagination field resolvers: Support for own types and validation

[WipeAir]

The current implementation of the pagination field resolvers requires the return type of a field to be a list (e.g. [User]). But the generator should also support own *Connection and *Edge types.
When bringing in their own types, the generator should validate them against the GraphQL Cursor Connections Specification.

The user should also be able to define additional fields on the *Connection and *Edge types. When additional fields are given the generator has to generate specific Kotlin classes to support those. (Currently, global Kotlin classes are being used (PaginationConnectionGenerator, PaginationEdgeGenerator))

The pagination field resolvers should also properly handle the nullability of the types.