Refactor merging + add abstraction for attributes

Recent PR discussion: https://github.com/hashicorp/terraform-plugin-codegen-openapi/pull/50#discussion_r1300194506

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: https://github.com/hashicorp/terraform-plugin-codegen-openapi/blob/aceac7a06052fbd85998365527b7e9b74e11d264/internal/mapper/attrmapper/types.go#L23-L41

hashicorp / terraform-plugin-codegen-openapi

Refactor merging + add abstraction for attributes #51

Notes