Consider consolidating vote-signing and VRF keys

[agustinmista]

Both the generic voting committee implementation and the initial Peras BLS intstantiation accept potentially different keys for the vote-signing and VRF-computing roles. This might be unnecessary complexity if we find later that both can always be the same.

There are two options when tackling this issue:

• removing the option of using different keys at the generic voting committee level, or
• removing the option of using different keys only for the concrete Peras instantiation.