[backstage] Add a Policy plugin

[foot]

Blocked by #3156 for now

Let's add a Policy view to Backstage!

Lets figure out what the developer-centric story here is.

Tasks

• Familiarise ourselves with the Policy feature, read through the user-guide and have a play around.
• Figure out a design and what elements are most useful for developers.
  • A: Policy violations
• [ ] Run through the backstage-cli ... new plugin flow to create a new plugin in https://github.com/weaveworks/weaveworks-backstage-enterprise (private one)
• Add a new panel or page to Backstage that will be useful to developers!

Question:

• Where will it live? Either in a new weave-gitops-plugin, or the weave-gitops-enterprise-plugin for now?
  • Answer!: The private plugin repo for now see #3156

[bigkevmcd]

We can maybe borrow some ideas from https://github.com/keptn-sandbox/backstage-plugin-keptn/tree/main over the design of the cards for this?