Return subfields in unpack error

[meparle]

Adding the capability to return subfield IDs from a composite type field within an UnpackError when there is an error unpacking a subfield.

It is necessary to provide this further detail for some card schemes as part of the format error information.

Adding a field FieldIDs to UnpackError to return the top level field ID plus an ID for each of however many levels of nested subfields error during unpacking.

The field FieldID could be deprecated once FieldIDs is present and sufficient notice given. Happy to add a deprecation notice if you would like.

[alovak]

what I suggest is to keep error wrapping you added, but instead of collecting the IDs in the wrapper methods, create a helper, that will iterate over the errors and extract all field IDs like this:

		fieldIDs := []string{}
		var unpackError *mooverrors.UnpackError
		for {
			if errors.As(err, &unpackError) {
				fieldIDs = append(fieldIDs, unpackError.FieldID)
				err = unpackError.Unwrap()
			} else {
				break
			}
		}

		fmt.Println(fieldIDs) // [3 2] - the same we expect in the tests

With such an approach, we keep UnpackError as it is, add error wrapping into a composite field and maybe a helper method without breaking changes.

What do you think?