Decouple External and ExternalManager components

[yiwen101]

Please confirm that you have searched existing issues in the repo

Yes, I have searched the existing issues

Any related issues?

This issue is a sub issue of the #2106

What is the area that this feature belongs to?

No response

Is your feature request related to a problem? Please describe.

As mentioned in #1206, External and ExternalManager components are highly coupled as External depends on ExternalManager and ExternalManger depends on External. This makes adding unit test and coming up with mock on them difficult.

Describe the solution you'd like

Refactor the code. This is a good first step towards achieve the goal of boosting test coverage to the core components.

Describe alternatives you've considered

No response

Additional context

No response