work with additional key values (#1757)

Add the possibility to inject additional key-values in the
sproof-builder that generates the relay root that gets stored in
parachain-system.

Rationale: pallets that verify additional storage items (not those
verified by parachain-system) from the relay should be able to proof
against the relay root that gets stored in parachain-system. This PR
allows to create provide additional nibles that can later be used for
verifiability in other pallets