Temporary local SetNestedAttributes implementation

[ewbankkit]

Relates #39.
Relates #54.
Until the Plugin Framework has a working SetNestedAttributes implementation, implement a local, temporary SetNestedAttributes type.
This should unblock unit tests for sets of objects.

[ewbankkit]

This will require a fork of terraform-plugin-framework to allow NestedAttributes to be implemented outside the schema package: https://github.com/ewbankkit/terraform-plugin-framework/tree/f-allow-nested-attributes-instantiation.

go mod edit -replace=github.com/hashicorp/terraform-plugin-framework=github.com/ewbankkit/terraform-plugin-framework@f-allow-nested-attributes-instantiation
go mod download github.com/hashicorp/terraform-plugin-framework