Backend/gsm and SDK upgrade

[knelasevero]

#91:

• Operator SDK updated to 1.0.1.
• OPERATOR_NAME not used in new controller-runtime, using LeaderElectionID instead
• deploy/ folder replaced with config/ which is handled by kustomize
  • CRD manifests in deploy/crds/ are now in config/crd/bases
  • CR manifests in deploy/crds/ are now in config/samples
  • Controller manifest deploy/operator.yaml is now in config/manager/manager.yaml
  • RBAC manifests in deploy are now in config/rbac/
• Go updated to 1.15
• Added tests to the externalsecret-controller
• Helpers and options to work with webhooks
• use multigroup: true incase we need to support more complex secrets
• Add config/backend-config to handle backend-config secrets
• Previous operator code in /legacy

#47

• Add Dockerfile build stage and use https://github.com/GoogleContainerTools/distroless - generated by operator-sdk

#3

• Introduce support for GCP secret manager(gsm) https://cloud.google.com/secret-manager/docs/configuring-secret-manager

#86

• Migrate to github actions from circle ci

Checklist

• ASM backend unaffected by upgrade
• Onepassword backend unaffected by upgrade
• Readme and contribution guide updated

Possible new issues

• Onepassword needs work
• Helm needs work

[riccardomc]

Impressive! There is SO MUCH in this PR 😄

Personally, I would split it in a few PRs, but hey... it's green... so, it's really up to you!

Well done!