Add secrets for pulling custom connector images

[xiaohansong]

What

Add secrets for pulling custom connector images

How

Adding an optional config field and pass it along to the pod creation process. Pod can accept them as a list and can pull images from both repositories if configured.

[pmossman]

I think instead of adding a brand new config, it would make more sense for our OSS users if we instead updated the existing config to support a list.

For an OSS user who wants to use custom images, they're supposed to use the existing config, so having a second config will add confusion.

Basically, we just need to add support for defining multiple image pull secrets, so that Cloud can use two private image repositories instead of being limited to one.

Does that make sense?

[xiaohansong]

Makes sense, good call! I'll fix this.

[pmossman]

Thanks! Sorry to make you rework it all 😅

[xiaohansong]

Fixed - and retested on dev, this should work! (though some other bugs were detected, PR coming in...)

[pmossman]

Looks good @xiaohansong! I left a few nits about making variable names plural. I was wondering if we should change the environment variable from JOB_KUBE_MAIN_CONTAINER_IMAGE_PULL_SECRET to JOB_KUBE_MAIN_CONTAINER_IMAGE_PULL_SECRETS but it might not be worth it since it'd be a breaking change for OSS deployments that use it (and we'd need to update the environment variables in Cloud too, though I'm more concerned with breaking changes in OSS)

I think renaming our code variables and method to be plural while leaving the environment variable alone is a good middle ground, at least in my opinion.