feat: add permission framework support

[drodil]

Add Backstage permission framework support to control who can post new / edit / see specific bulletins. See the docs from https://backstage.io/docs/permissions/plugin-authors/01-setup

[Shocktrooper]

Are there default permissions for this application by chance already in place? If so could they be documented somewhere

[v-ngu]

Unfortunately nothing is in place for the moment. I will have to work on it, unless you want to tackle the issue?