refactor: Add defaults for authZ policy

[ZohebShaikh]

It feels slightly awkward to have to specify the policy values explicitly. I think having sensible defaults would provide a better UX.

They could also be included in Copier, but having them as defaults would mean one less thing to worry about when running copier update.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.96%. Comparing base (2108ee0) to head (a2eb261).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1621   +/-   ##
=======================================
  Coverage   95.96%   95.96%           
=======================================
  Files          45       45           
  Lines        3317     3317           
=======================================
  Hits         3183     3183           
  Misses        134      134           

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[ZohebShaikh]

@shree-iyengar-dls If this change gets merged in Then the only docs that should be added is that please point it to the authorization provider which is opa.diamond.ac.uk