Reference: #70

This change set includes an initial implemention of type-based semantic equality functionality for the framework.

Semantic equality functionality enables provider developers to prevent drift detection and certain cases of Terraform data consistency errors, by writing logic that defines when a prior value should be preserved when compared to a new value with inconsequential differences. The definition of inconsequential depends on the context of the value, but typically it refers to when a values have equivalent meaning with differing syntax. For example, the JSON specification defines whitespace as optional and object properties as unordered. Remote systems or code libraries may normalize JSON encoded strings into a differing byte ordering while remaining exactly equivalent in terms of the JSON specification.

Type-based refers to the logic being baked into the extensible custom type system. This design is preferable over being schema-based, which refers to logic being repetitively coded across multiple attribute definitions. Being type-based means the provider developer can state these intentions by nature of pointing to a custom type which bakes in this logic, rather than needing to remember all the details to duplicate. For example, proper JSON string handling in the prior terraform-plugin-sdk required implementing an attribute as a string type with an appropriate JSON validation reference and JSON normalization reference. With these changes, a bespoke and reusable JSON string type with automatic validation and equivalency handling can be created.

Developers can implement semantic equality logic by implementing a new type-specific interface, which has one method that receives the new value and should return whether the prior value should be preserved or any diagnostics.

The website documentation for custom types has been rewritten to remove details more pertinent to the original design of the framework and instead focus on how developers can either use existing custom types or create custom types based on the `basetypes` package `Typable` and `Valuable` interfaces.