Consider Adding Terraform-specific Behaviour Properties

### Background

Previously, the codegen-spec schema included Framework-specific implementation details for Plan Modifiers, which have since been removed (refer to [Removing framework-specific implementation details from plan modifiers](https://github.com/hashicorp/terraform-plugin-codegen-spec/pull/84#top)).

### Proposal

We could consider modifying the codegen-spec schema to allow inclusion of Terraform-specific Behaviour Properties (e.g., "does not change on update") to provide an implementation-agnostic way of defining desired behaviours.

### References

- [Remove Framework-specific Implementation Details from Plan Modifiers](https://github.com/hashicorp/terraform-plugin-codegen-spec/issues/83#top)