Allow distinct operator per validator(s)

[mempirate]

Context

Currently, the sidecar assumes the same operator for all of the validators that connect to it, and will use the same key to sign commitment requests for all those validators. It would be better to be able to define an operator per validator set in case multiple operators are being used. In practice, this might not happen very often, so we should have sensible defaults. The only reason I see currently is diversification across restaking protocols and testing.