Feature request: stacking helper function for `verify-signer-key-sig`

[hstove]

Problem

When a client wants to verify a Stacking payload, they can validate the signature by re-generating the PoX-4 message hash. This works for users that include a signer-signature, but that param is optional. When the signature is omitted, the pox-4 contract checks the signer-key-authorization map to ensure that an authorization was previously set.

There's a helper read-only function in pox-4 verify-signer-key-sig which essentially does both of these things (either verify the signature or verify that an authorization exists). It would be nice to expose this read-only function in the Stacking library, so that clients can validate a payload before submitting a transaction.

Solution

Add a verifySignerKey() (or similarly named) function that calls the read-only function verify-signer-key-sig.