[Feature]: Support ExternalImage for NWB Schema 2.9.0

[rly]

What would you like to see added to PyNWB?

See https://github.com/NeurodataWithoutBorders/nwb-schema/pull/604

Is your feature request related to a problem?

No response

What solution would you like?

Add support for BaseImage and ExternalImage for NWB Schema 2.9.0

Do you have any interest in helping implement the feature?

Yes.

Code of Conduct

• [x] I agree to follow this project's Code of Conduct
• [x] Have you checked the Contributing document?
• [x] Have you ensured this change was not already requested?