Param signer_key should work without 0x prefix

[aryzing]

The /extended/v2/pox/cycles/:cycle_number/signers/:signer_key/stackers endpoint (docs) 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.