external services: Add a ToAPIService method

Created by: ryanslade

This just moves the logic into a more central place so that it's harder to forget to update the code when we add more fields.

Simpler approach to close https://github.com/sourcegraph/sourcegraph/issues/33997

It also avoid a second load from the db which is not free for external services as in some cases we need to decrypt config.

Test plan

No code changed, just moved.