[libgraphql-core] Switch all type-builders to a more standard ::new().build() API

[jeffmo]

Currently all of the type-builders in libgraphql-core use an interface somewhat uniquely designed for an older version of schema::SchemaBuilder. They are also not exposed publicly -- which means there's not a good API for procedurally constructing types to be passed into a SchemaBuilder iteratively in code (vs defining via GraphQL's IDL language).

To square out our public API a bit more, we need to:

1. Convert each type-builder in libgraphql_core::types::* to use a more conventional ::new() -> .build(self) API.
2. Update SchemaBuilder to use these updated type-builder APIs
3. Expose the new, more conventional builders publicly so that they can be passed into a SchemaBuilder directly