Move TrustedCallSigned definition to itp-stf-primitives and make it generic over TrustedCall

[clangenb]

I don't really understand why we have this weird relation between TrustedCall, and the trait TrustedCallSigning<TrustedCall> and that TrustedCallSigned. It would be way simpler to just have a TrustedCallSigned<TrustedCall> and construct t the same way substrate constructs extrinsics.

If we do it like that we can also move quite some logic from app-libs to core, which is definitely beneficial for downstream developers.