Feature Request: Only build/push modified examples with GitHub action

[t-fine]

Is your feature request related to a problem? Please describe.

Before converting to GitHub actions to build and test the examples, we would only build and publish the services that were included in the new PR, and when those services passed the test targets they were optionally published to a public management hub. Right now the GitHub action takes a very long time to complete. I would like to see if we can only build the examples modified by the incoming PR and optionally push the updated version to the Open Horizon Docker hub.

Describe the solution you'd like.

Only build the examples modified by the incoming PR and optionally push the updated version to the Open Horizon Docker hub.

[t-fine]

Get a list of changed files: https://github.com/marketplace/actions/changed-files