IfExists.RETURN_OR_UPDATE configuration for createOrUpdateArtifact

[famarting]

from #15

In order to evolve a schema and therefore to create new versions of one artifact currently we only support explicitly using the version property, like:
...
metadata:
name: pets-avro-example-v1
spec:
artifactId: pests-avro-record
...
...
metadata:
name: pets-avro-example-v2
spec:
artifactId: pests-avro-record
version: "2"
...

However if we use the feature IfExists.RETURN_OR_UPDATE when calling createArtifact new versions could be created automatically.

Test this carefully to avoid having a situation where new versions are continuosly created in a loop.

[dweber019]

I would suggest even make this configurable over env variables, so one can choose the behavior.