Param `signer_key` should work without `0x` prefix

The `/extended/v2/pox/cycles/:cycle_number/signers/:signer_key/stackers` endpoint ([docs](https://docs.hiro.so/api/get-stackers-for-signer-in-po-x-cycle)) requires the signer key to be provided with a `0x` prefix. To match the behavior in other Hiro tools when dealing with hex-encoded strings, such as Stacks.js, the API would accept a hex-encoded string without the `0x` prefix.