Make git connection optional for git dag bundle

[dstandish]

Make it so we don't need to define a git connection for a git dag bundle.

[ephraimbuddy]

Instead of making GitDagBundle not to require a connection, how about creating an entire class that people can use if they don't want to use connection? We can rename GitDagBundle to reflect that it requires a connection and name the new bundle to reflect that it doesn't need it. This will make GitDagBundle consistent with how we design operators or anything that needs a connection by keeping a default connection ID

[ashb]

I'm not too familiar with the bundle backends, but having a separate class seems like an odd UX decision.

One of the reasons for wanting to make it optional is so that you can either:

• Point your git bundle at a public repo for easy demo/onboarding; or
• Make use of an existing SSH key on your machine.

In some ways it's similar to the AWS Hook, where if you don't configure it it will try and use instance identity out of the box, or what ever other configs it can find on the host. Which is also how git CLI works. I'd say we should be similar to that.