Refactor merging + add abstraction for attributes

[austinvalle]

Recent PR discussion: #50 (comment)

This PR introduces a new package that wraps the codegen-spec types to allow our code to easily handle generic logic such as: merging, overriding, etc. This PR is all internal refactoring and no new features/oddities have been added.

Prior art considered: https://github.com/hashicorp/terraform-plugin-codegen-framework/tree/main/internal/resource_generate

Notes

• For nested objects + nested attributes, we shadow the original NestedObject and Attributes properties. It looks a little weird when creating a struct literal, but otherwise behaves normally:

singleNestedAttribute := ResourceAttributes{
	&ResourceSingleNestedAttribute{
		Name: "single_nested_attribute",
		// This is correct!
		Attributes: ResourceAttributes{
			&ResourceStringAttribute{
				Name: "string_attribute",
				StringAttribute: resource.StringAttribute{
					ComputedOptionalRequired: schema.Required,
				},
			},
		},
		SingleNestedAttribute: resource.SingleNestedAttribute{
			// This is being shadowed!
			// Attributes: resource.Attributes{},
			ComputedOptionalRequired: schema.Required,
		},
	},
}

• Element types don't have any considerations during merging/overriding, so I didn't create wrappers for them and just kept the original function for merging:

  terraform-plugin-codegen-openapi/internal/mapper/attrmapper/types.go

  Lines 23 to 41 in aceac7a

  	func mergeElementType(target schema.ElementType, merge schema.ElementType) schema.ElementType {
  	// Handle collection type
  	// https://developer.hashicorp.com/terraform/plugin/framework/handling-data/types#collection-types
  	if target.List != nil && merge.List != nil {
  	target.List.ElementType = mergeElementType(target.List.ElementType, merge.List.ElementType)
  	} else if target.Map != nil && merge.Map != nil {
  	target.Map.ElementType = mergeElementType(target.Map.ElementType, merge.Map.ElementType)
  	} else if target.Set != nil && merge.Set != nil {
  	target.Set.ElementType = mergeElementType(target.Set.ElementType, merge.Set.ElementType)
  	}
  	// Handle object type
  	// https://developer.hashicorp.com/terraform/plugin/framework/handling-data/types#object-type
  	if target.Object != nil && merge.Object != nil {
  	target.Object.AttributeTypes = mergeObjectAttributeTypes(target.Object.AttributeTypes, merge.Object.AttributeTypes)
  	}
  	return target
  	}

[bflad]

Looks great! 🚀

[github-actions]

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