feat: remove useAtChops references in code

[murali-shris]

- What I did

• changes to enforce atClient to use atChops for authentication
• remove the references to AtClientPreference.privateKey in tests
• remove the use of EncryptionUtil in few classes which was used as fallback if useAtChops=false
  - How I did it
• added a method _createAtChops in AtClientImpl. If atChops is not injected from at_client_mobile/at_onboarding_cli, then keys will be read from local secondary and atChops instance will be created.
• removed use of EncryptionUtil in shared_key_decryption.dart, abstract_atkey_encryption.dart
• removed code which reads from private key in monitor.dart
• deprecated useAtChops experimental flag in AtClientPreference. Removed all references in at_client
• removed call to SignInPublicData.signData in put_request_transformer.dart
• refactored unit tests to remove references to useAtChops and mocked atChops where required
• removed references to AtClientPreference.privateKey in tests
• added a method in functional tests TestUtils.initAtClient(atSign, namespace); This will create at_chops instance and set it in the call to AtClientManager.setCurrentAtSign and also removes duplicate code in setUpAll methods in functional tests
  - How to verify it
• unit and functional tests should pass
• sanity in apps/cli with this branch of at_client.
  @purnimavenkatasubbu Please take this up.

[gkc]

LGTM - thank you @murali-shris