Move `SparseField` implementations out of `endpoint` module

[oguzkocer]

SparseField implementations should be part of each individual type, as they are not related to endpoints.

• Move wp_api::endpoint::macros::default_sparse_field_implementation_from_field_name! to root level
• Move the callers of this macros out of endpoint module and into their relative modules. For example, out of users_endpoint module and into users module.