feat: implement threshold strategy

[gregorydemay]

Follow-up on #286 to implement the threshold strategy to aggregate responses from multiple providers:

• Introduce a top-level MultiCallResults::reduce method which takes as parameter a ConsensusStrategy to aggregate the various responses. All existing methods, including eth_fee_history, were refactor to use the new reduce method.
• In case of ConsensusStrategy::Threshold, the reduction strategy simply consists of returning the most frequent (non-error) response if it appeared at least the specified threshold number of times.
• Refactor MultiCallResults to stored ok results and errors separately.

[THLO]

if it appeared more that the specified threshold number of times.

Shouldn't it be "at least the specified threshold number of times"?