replace encryption util methods with at_chops

[murali-shris]

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

• All encryption/decryption operations in at_client should be replaced with at_chops

Describe the solution you'd like

• local_key_decryption.dart, self_key_decryption.dart, shared_key_decryption.dart
  EncryptionUtil.decryptValue
  replace with at_chops.decryptString
• shared_key_decryption.dart
  EncryptionUtil.md5CheckSum - investigate whether md5 has to replaced
• self_key_encryption.dart,shared_key_encryption.dart
  EncryptionUtil.encryptValue
  replace with at_chops.encryptString
• abstract_atkey_encryption.dart --> createMyCopyOfSharedSymmetricKey/
  EncryptionUtil.generateAESKey() - replace with AtChopUtil
• abstract_atkey_encryption.dart --> verifyTheirCopyOfSharedSymmetricKey
  EncryptionUtil.encryptKey() - explore which method in at_chops to use
• Uptake at_chops major version in dependent packages

Describe alternatives you've considered

No response

Additional context

No response

[murali-shris]

Completed first sub task .Merged and published at_client 3.0.71
#1185
Implementation done for replacing md5 checksum. Waiting for at_commons 4.x update in at_client to be merged.
Working on 3rd sub task. Writing unit tests for existing code before starting implementation

[murali-shris]

Additional effort required for uptaking at_chops major version changes in different packages

[murali-shris]

Uptake of at_chops major version
at_chops changes - atsign-foundation/at_libraries#494
at_auth - in progress. uptake_at_chops_major_version_atauth
at_server - draft PR
at_client - #1213
at_lookup - draft PR
at_onboarding_cli - In progress. uptake_atchops_major_version_atonboarding_cli
at_client_mobile - TBD

[murali-shris]

Uptake of at_chops major version done.
Implementation for replacing md5 in progress

[murali-shris]

taking up atsign-foundation/at_server#1788 first and then will make changes for md5

[murali-shris]

moving to backlog due to APKAM priorities