Add support for labels in managed secrets

[tommatime]

This adds the ability to specify labels to be synced to the managed secret. There is currently no way to ensure a label is added to the managed secret which is required, for example, to set the secret-type for ArgoCD resources. If omitted, the existing "secrets.doppler.com/subtype": "dopplerSecret" label is applied to the managed Secret. If labels are included, the existing subtype label will be appended to the user-supplied labels to ensure it is not overwritten.

Syntax:

apiVersion: secrets.doppler.com/v1alpha1
kind: DopplerSecret
metadata:
  name: dopplersecret-test
  namespace: doppler-operator-system
spec:
  tokenSecret:
    name: doppler-token-secret
  managedSecret:
    name: doppler-test-secret
    namespace: default
    labels:
      doppler-secret-label: test

Fixes #64

[tommatime]

This is my first contribution to this project (and my first OSS contribution in general 🥳 ), so please let me know if any additional updates or documentation changes are necessary.

A comment on the original issue also requests support for annotations on the managed secret. This should be very similar to labels, so let me know if this functionality is desired and I can add that to this PR as well.

Thanks!

[nmanoogian]

Thanks so much for your submission, @tommatime! And congrats on your first OSS contribution! 🎉

We'll take a look at this PR and get back to you with feedback shortly.