Extract `EncryptedString` into a separate interface package similar to `NServiceBus.MessageInterfaces`

[ramonsmits]

Describe the suggested improvement

Is your improvement related to a problem? Please describe.

Currently property encryption is limited to string values only. A user can inherit from EncryptedString and use that in its message types but this type results in coupling the message type to a specific version of NServiceBus.Encryption.MessageProperty.

Describe the suggested solution

Extract EncryptedString into a separate interface package similar to NServiceBus.MessageInterfaces so this type can be shared between multiple versions of NServiceBus.Encryption.MessageProperty.

Additional Context

No response