add release workflow for keda connectors

[suramya-shah]

The release workflow will be triggered when:

• there is a merge at the master branch and **/version file is changed.
  Following things are achieved in the workflow:
• builds the docker image and pushes it to the respective docker hub repository of the connector
• tags the image as latest, version no. mentioned in the file.
  Adding reference to 7.

[RealHarshThakur]

@suramya-shah I think it would be nice if you add a new section around the version file in the https://github.com/fission/keda-connectors/blob/master/CONTRIBUTING.md file .

[RealHarshThakur]

Also, we added a new connector for GCP recently. We should include that in this in PR too

[vishal-biyani]

Hey @suramya-shah One last request. We do already have images with v0.5 as tag. So we should bump the one in this PR to v0.6 so that old images are preserved and new one are tagged. I am assuming once we merge - this will build and push all images?

[suramya-shah]

yes @vishal-biyani , now it should.

[RealHarshThakur]

I think the GCP one should be 0.1 @suramya-shah