UftpValidator to give context specific rejection reasons

[tomwetjens]

Currently UftpValidator only supports 1 rejection reason (through getReason) which cannot be made context specific to the validated message.

Case 1

E.g. a user defined validator that checks the ExpirationDateTime must be before Period-12H.

Currently the reason would always be: "ExpirationDateTime must at least 12H before Period"

Desired reason: "ExpirationDateTime must be before YYYY-MM-DDTHH:mm:ss"

Case 2

E.g. a user defined validator that combines 2 checks and wants to give a specific reason for each of the cases. Currently this would require two separate validator classes that each must have the correct order.

Proposal:

Let isValid return a ValidationResult including the reason instead of a boolean. Remove getReason.