Support for PS256 algorithm

[ataylormays-duetto]

Describe the problem you'd like to have solved

According to both https://github.com/auth0/java-jwt#requirements and this library's profile on JWT.io (https://jwt.io/libraries?language=Java), there's no support for the PS256 algorithm whereas most other JWT libraries do support it.

[jimmyjames]

Seems like a good algorithm to support; while we might not get to this super soon, contributions are welcome! Some implementation considerations (I haven't looked into PS256 support in Java, but just in general):

• We should maintain java 8 compile compatibility
• We shouldn't add any additional security providers to the implementation dependencies