feat: Add enrollment setup and run tests with APKAM authentication

[sitaram-kalluri]

- What I did

• Modify the end-to-end tests to run with APKAM authentication.
• Add "revoke" method to enrollment_service.dart to support "enroll:revoke" operation.
• In AtClientImpl.dart, if the AtKey.namespace is null, we set the namespace from AtClientPreferences to the key. In collection framework, the namespace of the key is added to the "AtKey.key" and "AtKey.namespace" is unset. As a result the namespace from the "AtClientPreferences" is added causing duplicate/multiple namespaces being added to the key. Modified the code in "default_key_maker.dart" to fix the issue. Removed the namespace from the AtKey.key and added to AtKey.namespace.

- How I did it

• Add "enrollment_setup.dart" script that submits an enrollment request to the server and approves the enrollment request. Upon approval of the request, the authentication keys are written to the atKeys file which are used by the tests for authentication.
• The enrollment_setup.dart file is invoked by the github actions to generate at the atkeys file prior to running of tests.
• In Config14.yaml, introduce "authType" variables which is set to "apkam" to run authenticate at the atsigns via the APKAM authentication. The tests run between the trunk version of sever - @ce2e1 and canary version of server -@ce2e4. In Config12.yaml, the "authType" is set to "pkam" to retain the existing way of running the tests.
• In the "setUpAll" of each test, fetch the "authType" from the config.yaml to perform the authentication.
• Since "notify:text" is being deprecated, removed the tests related to it.
• In config14.yaml, the firstAtSign and secondAtSign are configured as "@ce2e1" and "@ce2e2" respectively, causing the tests to run against the trunk branch on both servers. However, the intended behavior of config14.yaml is to test against both the trunk branch and the canary version on the server. Therefore, updated the secondAtSign to "@ce2e4" and the fourthAtSign to "@ce2e2" to align with this intention.
• Add "enrollment_teardown.dart" file which will revoke/deny the approved/pending enrollments respectively after the running the tests.

- How to verify it

• All tests should pass.

- Description for the changelog

• Run E2E tests with APKAM authentication.

[sitaram-kalluri]

Currently, the E2E tests are running successfully. The unit tests are failing with dart beta version due to following dart issue: https://github.com/dart-lang/sdk/issues/55911#issuecomment-2153606884. Pending work is to update the dependencies and check if the issue is resolved.