fix: Introduce time duration for apkam keys to auto expire

[sitaram-kalluri]

- What I did

• Introduce APKAM keys to auto expire after specified time duration.

- How I did it

• In at_commons, introduced Duration apkamKeysExpiryDuration to capture the time duration set by the user. This time duration is propagated to the secondary server.
• In the secondary server, once enrollment is approved, set the TTL value to the Duration apkamKeysExpiryDuration in milliseconds. This will ensure that the key expires and is subsequently deleted from the keystore.
• In "AbstractVerbHandler.dart", add "_verifyIfEnrollmentIsActive", which will check if the enrollment is active on the existing connections. If the enrollment expires, closes the connection with error message.
• For the new connections, in the "PkamVerbHandler.dart->handleApkamVerification", there is call to "getEnrollDataStoreValue" which will check if the enrollment is active. If not active, sets enrollment state to expired and authentication fails.

- How to verify it

• Added a unit test to verify the changes.

  • A test to verify apkam expiry is set for approved enrollment
  • A test to verify pkam verb fails when apkam keys are expired
  • A test to verify update verb fails when apkam keys are expired
  • A test to verify delete verb fails when apkam keys are expired

• Added below functional tests:

  • A test to verify apkam authentication fails with expired apkam keys
  • A test to verify connection closes when apkam keys expire

- Description for the changelog

• fix: Introduce time duration for apkam keys to auto expire

[gkc]

A few things missing here:

1. Enforcement to prevent a client from authenticating with an enrollment that has expired
2. Test(s) to verify the same
3. Enforcement in the verb handler(s) to terminate connections whose enrollment has expired
4. Test(s) to verify the same

[sitaram-kalluri]

A few things missing here:

1. Enforcement to prevent a client from authenticating with an enrollment that has expired

2. Test(s) to verify the same

3. Enforcement in the verb handler(s) to terminate connections whose enrollment has expired

4. Test(s) to verify the same

@gkc : Addressed the review comments and updated the PR description. Please review.

[gkc]

Looks good. However, we need to be smarter in terms of dealing with enrollment records - checking every time is necessary, but means an extra fetch from the data store every time. We should create an EnrollmentManager class through which all interaction with enrollment records happens, and which can therefore make appropriate decisions regarding in-memory caching of enrollment records. Can you check the effort involved in doing that - if it's not huge then we should make those changes in this PR also.