Create per-resource functions to generate import id base64 data

[kklimonda-cl]

• Cast received types.Object into an actual resource Model structure
• Fill Import State struct with data from the model (location, names etc.)
• Marshal import state struct into JSON and return it

Figure out error handling between our code and terraform functions.

An example implementation for a simple Entry-type resource, although not complete - we need marshaling of Terraform Locations for it to work, and EncodeLocation() doesn't seem to work properly:

type AddressGroupImportStateStruct struct {
	Location AddressGroupLocation
	Name     string
}

func AddressGroupImportStateCreator(ctx context.Context, resource types.Object) (string, error) {
	var addressGroup AddressGroupDataSourceModel
	diags := resource.As(ctx, &addressGroup, basetypes.ObjectAsOptions{})
	if diags.HasError() {
		return "", fmt.Errorf("Failed to convert types.Object into Model")
	}

	importStruct := AddressGroupImportStateStruct{
		Location: addressGroup.Location,
		Name:     addressGroup.Name.ValueString(),
	}

	data, err := json.Marshal(importStruct)
	if err != nil {
		return "", err
	}

	return base64.StdEncoding.EncodeToString(data), nil
}

Uuid-style and Entry-style lists will require some tweaks: it would probably be enough to change Name string to Names []string and copy names from the map/list (depending on the resource type).