Allow for custom backend URIs

[linkognito]

The custom-backend and backend policies is currently tied to fixed backend paths (starting with /neos).
Custom backend-like routes (e.g. /monocle) can’t reuse it without patching.

Making the URI matching for custom-backend and backend configurable would allow applying the existing backend CSP to custom paths.

Possible configuration:

Flowpack:
  ContentSecurityPolicy:
    policies:
      custom-backend:
        matchUris:
          - '^/monocle(/.*)?$'

[skurfuerst]

@linkognito thanks so much for the issue :) If you like and if possible, feel free to prepare a Pull Request for this :)

All the best,
Sebastian