AtClient: Deprecate the (experimental) AtProtocolEmitted in prefs and replace with AtEncryptionScheme

[gkc]

Is your feature request related to a problem? Please describe.

• Encryption / decryption is a client-side activity
• i.e. it does not relate to AtProtocol per se
• so the name is misleading

Describe the solution you'd like

• Deprecate the (experimental) AtProtocolEmitted in AtClient prefs and replace with AtEncryptionScheme
• Once complete, create tickets to replicate in the Java, Python and C SDKs

Describe alternatives you've considered

No response

Additional context

• In the medium term we will need to refactor our client SDKs so that data Encryption and data Decryption implementations can be added by client code and invoked based on the corresponding encryption-related metadata for a given data record
• Intention being that users should be able to use our AtClient libraries in whatever language while being able to plug in their own encryption / decryption schemes with minimal effort

[gkc]

Will complete in PR80 if I can find two spare hours some day