Support image config git polling ignore certain directories

[mgibson1121]

Motivation: This is feedback from @samarsinghal. In Gitops based workflows, the source as well as deployment configuration often live in the same repository. If kpack is monitoring this git repository for changes, and the user updates the digest specified in the deployment manifest (either manually or within a pipeline), the kpack image will pick up the change and execute an undesired build.

The alternative is to better segment processes for building and deployment, but the feedback provided states that this is at odds with the Gitops model of keeping deployments in the same repo.

Requested Solution: Provide some mechanism to exclude directories from the git source trigger process in the image resource. This way, if both source and deployment configuration lived together in the same project, devs could make updates to the deployment directories without triggering rebuilds.