Added error message for macOS not supported. Resolved compiler warnings

E2ETests/SenderTest.cs

Encryption/JoseEncryptor.cs

FitConnect/FluentSender.cs

FitConnect/Models/Destination.cs

FitConnect/Models/Submission.cs

Services/Models/CallbackDto.cs

Services/Models/OAuthAccessToken.cs

Services/Models/v1/Case/EventLogDto.cs

Services/Models/v1/Destination/ContactInformationDto.cs

Services/Models/v1/Destination/CreateDestinationDto.cs

Services/Models/v1/Destination/DestinationListDto.cs

Services/Models/v1/Destination/DestinationServiceDto.cs

Services/Models/v1/Destination/PatchDestinationDto.cs

Services/Models/v1/Destination/PrivateDestinationDto.cs

Services/Models/v1/Destination/SubmissionSchemaDto.cs

Services/Models/v1/Destination/UpdateDestinationDto.cs

Services/Models/v1/Submission/SubmissionCreatedDto.cs

Services/Models/v1/Submission/SubmissionDto.cs

Services/Models/v1/Submission/SubmissionForPickupDto.cs

Services/Models/v1/Submission/SubmissionReducedDto.cs