types: add key option to sign and verify

[KirillTregubov]

Closes #313

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[gurgunday]

Can you add a test for the type?

[KirillTregubov]

I have added tests to the jwt.test-d.ts file.

Interestingly, while looking around in the codebase, I could not find a justification for the onlyCookie definition on the VerifyOptions type. It is not documented in the README, and onlyCookie does not seem to be used in the verify function inside jwt.js. Please let me know if this is safe to remove.

[mcollina]

lgtm