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

deps: updated

Closes #9

See merge request !102

Issue/717 validate submission data

See merge request !63

# Conflicts:
#	Examples/ConsoleAppExample/SenderDemo.cs

Issue/1072 service type getter

See merge request !81

# Conflicts:
#	Examples/ConsoleAppExample/SubscriberDemo.cs

Feature/1618 content length

See merge request !98

Feature/1308 attachment from string

See merge request !91

# Conflicts:
#	FitConnect/FitConnect.csproj
#	Tests/BasicUnitTest/BasicUnitTest.csproj
#	Tests/E2ETest/E2ETest.csproj
#	Tests/EncryptionTests/EncryptionTests.csproj
#	Tests/IntegrationTests/IntegrationTests.csproj
#	Tests/IntegrationTests/Routing/RoutingTests.cs

# Conflicts:
#	Examples/ConsoleAppExample/ConsoleAppExample.csproj
#	FitConnect/ClientFactory.cs
#	FitConnect/FitConnect.csproj
#	Tests/BasicUnitTest/BasicUnitTest.csproj
#	Tests/BasicUnitTest/SubscriberTests.cs
#	Tests/E2ETest/E2ETest.csproj
#	Tests/EncryptionTests/EncryptionTests.csproj
#	Tests/IntegrationTests/IntegrationTests.csproj
#	Tests/IntegrationTests/Routing/RoutingTests.cs
#	Tests/MockContainer/MockContainer.csproj