GitHub Action to create and push images to Google Artifact Registry

[devdattakulkarni]

We should add a GitHub Action that will build the image for each component and push it to GCR (where the official images are stored). This GH Action will be manually triggered.

The workflow will be:

1. Update versions.txt for relevant components; create PR; merge PR
2. Trigger build_and_push_images GH Action. This GH Action will build versioned images for each component and push them to GCR
3. Create new PR with updates to Chart.yaml and values.yaml in ./deploy/kubeplus-chart; merge PR
4. Create new KubePlus chart; push it to operatorcharts repository.
5. Create PR with updates to README and steps.txt in various examples to refer to the new chart tgz

[devdattakulkarni]

References:
https://github.com/marketplace/actions/push-to-gcr-github-action
https://medium.com/mistergreen-engineering/uploading-a-docker-image-to-gcr-using-github-actions-92e1cdf14811
https://medium.com/@sbkapelner/building-and-pushing-to-artifact-registry-with-github-actions-7027b3e443c1