Please add external_content to the supported zendesk APIs

[dima-patreon]

Currently I cannot find any support from zenpy for the zendesk external_content APIs https://developer.zendesk.com/api-reference/help_center/federated-search/external_content_sources/

I know they aren't the best APIs, but would be nice to have the utilities of zenpy like automated retries on 429s while using those endpoints.