keyvault commands to sign and verify signatures

**Is your feature request related to a problem? Please describe.**
I have a key (asymmetric, RSA) stored in Azure Key Vault. The key is used by an application to sign messages and verify digital signatures.

**Describe the solution you'd like**
I would like a command in the Azure CLI that requests AKV to sign a message, and another one that asks AKV to verify the message. This should happen within AKV, without the private key being exported (assuming the account has permissions to "sign").

**Describe alternatives you've considered**
I don't believe there's a CLI to do this. The alternative would be to build my own application.