Bug/1634 proxy

See merge request !99

# Conflicts:
#	Tests/E2ETest/StraightForwardTest.cs

Feature/1243 fachliches datum

See merge request !104

# Conflicts:
#	FitConnect/Builder/SubmissionBuilder.cs
#	FitConnect/SendableSubmission.cs
#	FitConnect/Sender.cs
#	Tests/E2ETest/StraightForwardTest.cs

# Conflicts:
#	Examples/ConsoleAppExample/SenderDemo.cs
#	FitConnect/Encryption/JsonHelper.cs
#	FitConnect/FitConnect.csproj
#	FitConnect/FitConnectClient.cs
#	FitConnect/Router.cs
#	FitConnect/Services/Interfaces/ISubmissionService.cs
#	Tests/BasicUnitTest/BasicUnitTest.csproj
#	Tests/E2ETest/AutoRejectReplyChannel.cs
#	Tests/E2ETest/E2ETest.csproj
#	Tests/E2ETest/RejectSubmissionTest.cs
#	Tests/E2ETest/StraightForwardTest.cs
#	Tests/EncryptionTests/EncryptionTests.csproj
#	Tests/MockContainer/MockContainer.csproj

Epic/1256 bidiko

See merge request !94

Additional method summaries have been introduced to provide more detailed code documentation. Refactoring in Sender.cs has also improved null handling in the methods ListAvailableRepliesAsync and AcceptReplyAsync. Moreover, minor adjustment has been made in Subscriber.cs for better variable declaration.

Made changes to improve null safety and overall code quality. Updated the nullability of certain properties across multiple classes to better reflect possible states. Refactored the constructors of the FitConnectClient class to avoid duplication, increasing the code's maintainability.

# Conflicts:
#	Examples/ConsoleAppExample/ConsoleAppExample.csproj
#	Examples/ConsoleAppExample/SenderDemo.cs
#	FitConnect/Encryption/FitEncryption.cs
#	FitConnect/FitConnect.csproj
#	FitConnect/Models/Attachment.cs
#	FitConnect/Models/SET/AuthenticationTags.cs
#	FitConnect/Subscriber.cs
#	Tests/BasicUnitTest/BasicUnitTest.csproj
#	Tests/BasicUnitTest/SecurityEventTokenTests.cs
#	Tests/E2ETest/E2ETest.csproj
#	Tests/EncryptionTests/EncryptionTests.csproj
#	Tests/IntegrationTests/IntegrationTests.csproj
#	Tests/MockContainer/MockContainer.csproj
#	Tests/ValidationTests/ValidationTests.csproj