Consider making `service_account_email` part of the `Signing` interface

[theacodes]

google-cloud-python needs both a signer and a service account email to identify the signer to make a signed url.

/cc @dhermes

[dhermes]

👍 This is a no-brainer?

[theacodes]

Should it be service_account_email? Should it just be signer_email?

[dhermes]

I don't know. You seem to have read the RFC?

[theacodes]

This is pretty removed from the RFC's scope. JWT has the concept of an issuer.