Move credentials endpoints to ecs-agent module

[amogh09]

Summary

Move v1 and v2 TMDS credentials endpoint handlers from agent module to ecs-agent module.

Implementation details

• Moved v1 and v2 credentials endpoint handlers from agent module to ecs-agent module.
  • agent/handlers/v1/credentials_handler.go to ecs-agent/tmds/handlers/v1/credentials_handler.go.
  • agent/handlers/v2/credentials_handler.go to ecs-agent/tmds/handlers/v2/credentials_handler.go.
• Added new unit tests to ecs-agent module for both credentials endpoint handlers. Although agent module has TMDS tests covering these endpoints, we want to have unit tests in ecs-agent module that exclusively cover these handlers too.
• Moved GetCredentialsEventType function from agent/logger/audit/entry_types.go to ecs-agent/logger/audit/audit_log.go. This function returns an appropriate audit log event type for the provided role type (task role or execution role) and is used by credentials endpoint handlers too. Also moved the corresponding audit log event type constants that this function returns.
• Moved ZeroOrNil util function from agent/utils/utils.go to ecs-agent/utils/utils.go as it is used by credentials handlers.

Testing

In addition to new unit tests, also performed manual stress testing for the two changed endpoints. There is no obvious change in performance seen.

New tests cover the changes: yes

Description for the changelog

Move v1 and v2 TMDS credentials endpoints to ecs-agent module.

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.