add a signer stakers endpoint filtered by cycle

Right now we have `/extended/v3/staking/signers/{principal}/stakers` that gives the current set of stakers. For some, it would be useful to be able to see past history of stakers in previous PoX cycles.

Start by tracing the existing /extended/v3/staking/signers/{principal}/stakers endpoint and how it returns the current staker set. Add a cycle filter so callers can retrieve stakers from previous PoX cycles, then verify that the endpoint returns the expected historical set for the requested cycle.

Written by the indexing model from the issue text.