Typescript JWK Key return type issues

Describe the bug Currently all return values regarding Keys are typed under the Key interface.

This causes that no Key type specific parameters can be accessed (for example EC Key crv value) in the return values regarding keys.

*To Reproduce

// fill with EC type public / private key
const ecTypeKey = 'EC KEY';
const key = JWK.asKey(ecTypeKey);
// This will cause an error: Property 'crv' does not exist on type 'Key'.
console.log(key.crv);

Expected behaviour Should return proper Key sub type, so parameters can be accessed

Environment:

• jose version: [e.g. v1.22.2]
• node version: [e.g. v13.3.0]

Additional context

• This should be implemented to all return values regarding Key type (JWE.completeDecrypt, etc)

[panva]

This branch will land with the next 13.x node release. Planned for the 18th

[HiskiRobo]

Nice solution in the commit! This can be closed. Thanks!