Create new methods signRequest and signResponse

[aljones15]

Currently in order to sign a request 2 different functions are called:

createAuthzHeader

createSignatureString

I think we should create a new function: signRequest and signResponse that uses these 2 functions, and a derefernced key to sign the request or response.

This would also allow us to do more validation when signing while knowing the algorithm and keyType.

While not as dire, verifyRequest and verifyResponse might be similarly useful and could wrap around parseRequest.