Allow Opaque types

[haerdib]

Probably connected to #406 . Don't solve this before that issue.

The problem: If one does not know the extrinsic types, substrate offers the OpaqueExtrsinic type, which deserializes correctly even though the "correct" Type isn't known (see https://github.com/integritee-network/integritee-node/blob/master/runtime/src/lib.rs#L108). However, currently it's not possible to make use of that because Runtime::xxx always takes the non opaque types.