Relax resource identity validation

[dbanck]

This PR removes two resource identity validations:

1. That an identity hasn't changed. Some resources have mutable identities, and the provider SDK can provide better safeguards and opt-out behaviour than enforcing this for every resource type in Terraform.
2. That an identity can't contain null values. Some identities need some sort of "optional" attribute, so we won't enforce that values must be non-null. This shifts some responsibility to provider authors that a resource identity is still uniquely identifiable even with null values.

Target Release

1.12.x

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[github-actions]

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