Creates Backward Compatible Context Class

[muditchaudhary]

Description

1. Currently, CedarJava handles Context as Map<String,Value> without a dedicated class. This PR creates Java equivalent class for Context.
2. Implements methods to create and merge Contexts within CedarJava. Currently, does not implement from_* methods.
3. Uses method overloading to make it backward compatible in authorization requests.
4. Implements tests for Context.
5. Implements tests to verify backward compatibility in AuthorizationRequest and PartialAuthorizationRequest.

Future Consideration:

• Create .copy() for all classes that implement Value interface. This will allow us to create deep-copy of any object that use Value. Currently, we can only create shallow copies for objects such as Context. For example, .getContext() creates and returns a shallow copy of the Map<String, Value> context but the Value object can potentially be modified.