introduce MakeVariantWithSignature

[phoracek]

Allow user to explicitly specifiy variant signature rather than
autodetect it.

[jsouthworth]

I think this is fine. Out of curiosity, why do you need this?

[phoracek]

I use D-Bus to control NetworkManager. Reason of this patch is to move duty of value retyping from user to dbus library.

I know that a parameter for D-Bus Call is expected to be uint32. But I don't want my user to explicitly type each value or convert it from int32 to uint32. My goal is to provide native interface, D-Bus calls are hidden.

Let's say user passed 1 to a function that sends this value to D-Bus Call which expects type u.
a) If I use MakeVariant(1), SignatureOf(1) correctly returns i and D-Bus call fails. In this case it would be required for user to pass uint32(1).
b) If I use MakeVariantWithSignature(x, ParseSignatureMust("u")) int32 value will be accepted by D-Bus.

Hope it makes sense. It's a little bit hacky, but it works for me.