all: Add support for embedded struct types in object conversions

[austinvalle]

Closes #242

Previous proposals: #941, #667

This PR introduces support for types.Object conversions with Go structs that utilize struct embedding:

• https://go.dev/ref/spec#Struct_types
• https://go.dev/doc/effective_go#embedding

This PR supports N levels of nested embedded struct types and applies the same tfsdk tag restrictions/validations regardless of where the tag appears in the struct.

Example

type thingResourceModel struct {
    Attr1 types.String `tfsdk:"attr_1"`
    Attr2 types.Bool   `tfsdk:"attr_2"`
    CommonModel
}

type CommonModel struct {
    Attr3 types.Int64 `tfsdk:"attr_3"`
    Attr4 types.List  `tfsdk:"attr_4"`
    EvenMoreCommonModel
}

type EvenMoreCommonModel struct {
    Attr5 types.String `tfsdk:"attr_5"`
    Attr6 types.Map    `tfsdk:"attr_6"`
}

func (r *thingResource) Schema(ctx context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
    resp.Schema = schema.Schema{
        Attributes: map[string]schema.Attribute{
            "attr_1": schema.StringAttribute{
                Required: true,
            },
            "attr_2": schema.BoolAttribute{
                Computed: true,
                PlanModifiers: []planmodifier.Bool{
                    boolplanmodifier.UseStateForUnknown(),
                },
            },
            "attr_3": schema.Int64Attribute{
                Required: true,
            },
            "attr_4": schema.ListAttribute{
                ElementType: types.StringType,
                Computed:    true,
                PlanModifiers: []planmodifier.List{
                    listplanmodifier.UseStateForUnknown(),
                },
            },
            "attr_5": schema.StringAttribute{
                Required: true,
            },
            "attr_6": schema.MapAttribute{
                ElementType: types.BoolType,
                Computed:    true,
                PlanModifiers: []planmodifier.Map{
                    mapplanmodifier.UseStateForUnknown(),
                },
            },
        },
    }
}

The same example schema could also be represented by these model combinations:

type thingResourceModel struct {
    Attr1 types.String `tfsdk:"attr_1"`
    Attr2 types.Bool   `tfsdk:"attr_2"`
    CommonModel
    EvenMoreCommonModel
}

type CommonModel struct {
    Attr3 types.Int64 `tfsdk:"attr_3"`
    Attr4 types.List  `tfsdk:"attr_4"`
}

type EvenMoreCommonModel struct {
    Attr5 types.String `tfsdk:"attr_5"`
    Attr6 types.Map    `tfsdk:"attr_6"`
}

Notes

• Additional test coverage: Add reflection tests for object models with tfsdk tags in framework providers terraform-provider-corner#263
• To avoid having to refactor a good chunk of our reflection logic, we explicitly are not supporting struct embedding with pointers. That doesn't stop us from adding support in the future if it's deemed necessary (although supporting unexported struct embeds from pointers I don't think we can do)
  • The encoding/json library supports exported struct embedding via pointers, but it was also historically relying on a bug in Go that allowed setting unexported struct embedding with pointers to work. When that bug was fixed, they chose not to completely remove pointer struct embedding for compatibility reasons.
• This PR could be considered a change in behavior, as it adds support for unexported embedded structs which promote exported fields
  • These would be previously ignored, but now could potentially raise an error diagnostic if tfsdk ignore tags are not included

type thingResourceModel struct {
    Attr1 types.String `tfsdk:"attr_1"`
    Attr2 types.Bool   `tfsdk:"attr_2"`
    existingModel
}

type existingModel struct {
    FieldNotRelated string // Needs a tfsdk tag now!
}

• You can ignore an entire embedded struct via the typical ignore tag

type thingResourceModel struct {
    Attr1 types.String `tfsdk:"attr_1"`
    Attr2 types.Bool   `tfsdk:"attr_2"`
    existingModel      `tfsdk:"-"` // Ignored!
}

type existingModel struct {
    FieldNotRelated string
}

[SBGoods]

Great job, Austin! Just one comment about empty struct tags.