support helm chart upgrades OOTB

[jstrachan]

with the old jx step create pullrequest chart we baked in the knowledge of how to upgrade a helm version change in source code.

it might be nice to add helm as a change option in the .jx/updatebot.yaml file format so folks could upgrade helm charts without worrying about the files to match + the regex stuff.

e.g. here's the code...
https://github.com/jenkins-x/jx/blob/master/pkg/cmd/step/create/pr/step_create_pr_chart.go#L94

so we could maybe enhance the config file: https://github.com/jenkins-x/jx-cli/blob/master/.jx/updatebot.yaml

to allow

apiVersion: updatebot.jenkins-x.io/v1alpha1
kind: UpdateConfig
spec:
  rules:
    - urls:
        - https://github.com/jenkins-x/jxr-versions
      changes:
        - chart: 
            name: my-chart

[salaboy]

for reference: https://github.com/jenkins-x/jx-cli/blob/master/.lighthouse/jenkins-x/release.yaml#L348-L351