Get state root from commitment - Signal Service

[LeoPatOZ]

Currently the signal service relies on the commitment (stored in the commitment store) to be a state root, however in reality commitments might in fact be a block hash or a hash for two values (which might include the state root).

We have no mechanism to extract a state root in the case the commitment is something other than a state root, therefore our current implementation might not work.

I see two options:

1. Make the verify signal function more generic as to what a commitment is, perhaps have a helper function to get the state root from the commitment (this might require additional info)
2. Decide now what a commitment is, this might impact how we verify signals

I think i prefer option 1 so we can remain unopinionated as to what a commitment is although finding a generic impl might be hard.