Add generic types to create functions

[avin-kavish]

I think it would be good to have these functions generically typed. If we know the shape of the payload, we can pass it here and get better type completion.

createSigner
createVerifier  

Would you accept a PR for this?

[simoneb]

Absolutely! Please remember to include tests for the types in https://github.com/nearform/fast-jwt/blob/master/test/types.spec.ts

[simoneb]

@avin-kavish any update on this?

[avin-kavish]

yeah, I did some work few months back. It's in #258

I don't think it's complete though.

[simoneb]

Thanks. Are you planning to complete it?

[avin-kavish]

yeah, at some point.