added callback validation method and tests

...c/main/java/dev/fitko/fitconnect/api/services/Sender.java

...in/java/dev/fitko/fitconnect/api/services/Subscriber.java

.../fitconnect/api/services/crypto/MessageDigestService.java

...fitconnect/api/services/validation/ValidationService.java

...c/main/java/dev/fitko/fitconnect/client/SenderClient.java

...in/java/dev/fitko/fitconnect/client/SubscriberClient.java

...st/java/dev/fitko/fitconnect/client/SenderClientTest.java

...ava/dev/fitko/fitconnect/client/SubscriberClientTest.java

...main/java/dev/fitko/fitconnect/core/SubmissionSender.java

.../java/dev/fitko/fitconnect/core/SubmissionSubscriber.java

...in/java/dev/fitko/fitconnect/core/crypto/HashService.java

.../fitconnect/core/validation/DefaultValidationService.java

...ava/dev/fitko/fitconnect/core/crypto/HashServiceTest.java

...connect/core/validation/DefaultValidationServiceTest.java