all: Add UseNonNullStateForUnknown plan modifier to assist with child nested attribute use-cases

[austinvalle]

Related Issue

Closes #1197
Ref #1211, #1161, #1117

Description

This PR introduces a new plan modifier that implements the same logic as pre-1.15.1 UseStateForUnknown, so it will not preserve null state values, allowing them to stay marked as unknown.

The primary use-case for a plan modifier like this is for use on child-attributes in a nested attribute which will only be preserved once the nested object exists (regardless of whether the resource has been created or not). See #1197 (comment) for a detailed description of the rationale.

While this plan modifier solves the problem more broad then needed, it's also the most accessible/understandable solution for nested attributes of requiring this behavior, since detecting if a parent object exists is not an easy operation (see any issue referencing set identity or their planning 😏 ).

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As there were existing providers that may be expecting pre 1.15.1 behavior on UseStateForUnknown, I plan on retroactively updating the 1.15.1 changelog (in this PR) + GH release to reference this behavior change more explicitly.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No

[austinvalle]

Also updated in the GH release (if we want to change the wording I can re-update 👍🏻 )
https://github.com/hashicorp/terraform-plugin-framework/releases/tag/v1.15.1

It felt weird to update the changelog to reference a future release, so I just mentioned the issue 🙂

[austinvalle]

For reference, the pre-v1.15.1 implementation: https://github.com/hashicorp/terraform-plugin-framework/blob/v1.15.0/resource/schema/stringplanmodifier/use_state_for_unknown.go