Add support for installing profiles from a tag

[aclevername]

What feature/behavior/change do you want?

Placeholder, may change depending on the output of #76

Follow up to #76, now we have documented and understand our approach for tagging we need to add support for getting a profile from a tag. Currently a subscription looks like:

apiVersion: weave.works/v1alpha1
kind: ProfileSubscription
metadata:
  name: nginx-profile-test
  namespace: default
spec:
  profileURL: https://github.com/weaveworks/nginx-profile
  branch: main

We use the profileURL and branch to fetch the profile. This story is to introduce a new alternative to the branch field, tag

apiVersion: weave.works/v1alpha1
kind: ProfileSubscription
metadata:
  name: nginx-profile-test
  namespace: default
spec:
  profileURL: https://github.com/weaveworks/nginx-profile
  tag: nginx/1.2.3

Why do you want this feature?
So I can install a profile from a tag