fix: updated to latest core SDK packages; miscellaneous fixes, enhancements and doc improvements

[gkc]

- What I did
Dependencies

• Updated pubspec to use latest core SDK packages
  • Took up version 3.0.57 of at_client for bug fixes and encryption enhancements
  • Took up version 1.2.5 of at_onboarding_cli so the keys files go in the correct location
• Updated pubspec to use latest versions of other packages

Bug fixes

• fix: convert received notification key to lower-case, and compare with lower-case value 'privatekey'
• fix: changed comparison for sshPublicKey also, to sshpublickey
• docs: Added code comments explaining why the toLowerCase is now being done, and why we are now comparing with lower-case values

Lint

• Eliminated most use of deprecated methods. (One exception: sync's onDone parameter)
• Eliminated all other lint warnings
• Fixed typos in code comments
• docs: Fixed typos and format issues in README.md
• Ran dart format on the codebase

fixes #100
fixes #102
fixes #103

[gkc]

@cconstab pushed some more changes

• build: Updated version in pubspec.yaml to 3.0.0
• build: Take up version 3.0.57 of at_client for bug fixes and encryption enhancements
• build: Take up version 1.2.5 of at_onboarding_cli so the keys files go in the correct location

If your testing on this branch goes OK (whenever you get a chance to do that) then this branch can be merged and we can publish release 3.0.0

[cconstab]

Extensive testing and a couple or minor log file changes and lower casing of the sshnp atKeys and we are good to go.. Thanks @gkc