feat: support for automatically initializing squashed pointer structs

[tuunit]

Description

This PR introduces support for automatically initializing squashed pointer structs during decodeStructFromMap.

Previously, when decoding into a struct with squashed embedded pointer fields, the decoder would fail if the pointer fields were not pre-initialized. This change ensures that such pointer fields are automatically initialized if they are nil, allowing for seamless decoding without requiring manual initialization.

• Improves usability by removing the need to pre-initialize squashed pointer structs. Especially useful for nested or dynamic structs and lists.
• Maintains backward compatibility with existing behavior for non-pointer and pre-initialized fields.

Issue

	type EmbeddedPointerSquash {
		*Basic `mapstructure:",squash"`
	}

	input := map[string]interface{}{
		"Vstring": "foo",
		"Vunique": "bar",
	}

	result := EmbeddedPointerSquash{} // Without initializing the internal struct
	err := Decode(input, &result)
	if err != nil {
		t.Fatalf("got an err: %s", err.Error())
	}

Would throw the error unsupported type for squash: ptr