feat: Implement RateLimiter into enroll_verb_handler and add unit test

[sitaram-kalluri]

- What I did

• Implement Rate limiting for the enrollment requests. Limit the enrollment request per connection to the configured value in the given time frame. An exception will be thrown for the enrollment requests received beyond the limit.

- How I did it
Changes in at_server_spec

• Create an abstract class called AtRateLimiter that includes a method called isRequestAllowed. This method should return a boolean value based on the rate limit rules.
• Modify the InboundConnection class to implement the AtRateLimiter abstract class.

Changes in at_secondary_server

• In the config.yaml file and at_secondary_config, introduce two new parameters: maxRequestsPerTimeFrame and timeFrameInHours to define rate limit criteria.
• Implement the isRequestAllowed method in the InboundConnectionImpl class (which implements InboundConnection) to enforce rate limits on enrollment requests based on the criteria specified in the config.
• In the enroll_verb_handler's _handleEnrollmentRequest method, call the isRequestAllowed method on the AtConnection object. If the method returns true, allow the enrollment request; otherwise, throw an AtThrottleLimitExceeded exception.
• Enhance the GlobalExceptionHandler to handle the AtThrottleException. This exception should be used to send a response to the client while preventing the connection from being closed.
• For writing function tests, when testingMode parameter in the config.yaml is set to true, allow modifications to the enrollment criteria using the config verb.
  • In the at_secondary_config, add maxRequestsPerTimeFrame and timeFrameInMills parameters to the ModifiableConfigs enum.
  • When the above values are modified through the config verb, implement listeners in the at_secondary_server_impl to capture these changes and update the values in the at_secondary_config.

- How to verify it

• Implemented unit tests in inbound_connection_impl_test.dart to validate the rate limit functionality on the inbound connection.
• Implemented functional tests to ensure that the rate limit is enforced on the enroll verb.

- Description for the changelog

• Implement Rate Limiting for APKAM requests

[gkc]

LGTM, thank you for the clear PR description @sitaram-kalluri

Once you have published at_server_spec and removed the dependency override from this PR please re-request review and I will approve and merge.