Add BaseDataSourceModel and BaseDataSource structs with refactored functions (#81)

* Add BaseDataSourceModel and BaseDataSource structs with refactored functions

These functions come from the different DataSource entities defined in
other files. We saw an opportunity to refactor these repeated functions
and common fields out to a new base struct.

* Add defensive programming to the BaseDataSource methods

Checks for:
- Contexts being active
- resp and resp.Diagnostics pointers being not nil
- HTTP Client defined before using it

* Add BaseEntityAPIModel to contain entity base fields

* Rename BaseDataSourceModel to BaseDataSourceModelWithOrg

* Rework the base models for DataSource definitions

* Add a ConfigValidators overload to BaseDataSourceWithOrg

* Add Variables field back due to backwards compatibility w/Deprecation

* Fix inventory_resource_test.go

Author: fincamd