Add NestedAttributes to provider functions

[jason-johnson]

Use-cases

Writing provider functions which have nested data type parameters like data sources, resources and providers can.

Attempted Solutions

Currently provider functions can only support this use case with DynamicAttributes.

Proposal

Allow provider functions to also use NestedAttributes to make defining more complex parameters easier the way it is in resources, etc.

[austinvalle]

Hey there @jason-johnson 👋🏻 ,

There isn't anything we can do on the SDK-side to implement the equivalent of a NestedAttribute, where you can define additional metadata for objects, since provider functions in the protocol aren't based off of a schema (like managed resources/data sources). Function parameters only have a type to work with, whereas resources built with a schema have a nested_type for defining that metadata.

The reason for this is that provider functions are built off, Terraform core's type system, go-cty Functions. For something similar to nested attributes to be possible, Terraform core would have to re-architect how provider functions work, which is unlikely at this point.

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Working with what we have, we could introduce type support for objects with optional attributes (from #955 (comment)), which would give you one aspect of nested attributes, which can determine which attributes are required from an object. This would be analogous to the optional support for object types in variable constraints: https://developer.hashicorp.com/terraform/language/expressions/type-constraints#optional-object-type-attributes

[austinvalle]

I've updated #955 to accurately reflect what is possible ATM, closing in favor of that