Tidy schema helper and add tests

[benPearce1]

This PR does the following:
Removes the util/schema.go file and moves all schema functionality to the schemas package
Introduces an interface for all schema defintions: EntitySchema
Implements that schema on all resource/datasource schemas
Adds a test in the ensure that all attributes are coming from the appropriate source schema package
e.g all attributes and nested attributes for a datasource come from github.com/hashicorp/terraform-plugin-framework/datasource/schema and vice-versa for resources