Update authz_proxy example to support fine-grained auth on Storage Backends

[j616]

Details

• Update authz_proxy example to support fine-grained auth on Storage Backends
• Fix incorrect query param name in filtered_object()

NOTE: This PR will not be merged until it has been tested against an implementation with the spec changes in #229

Issue (if relevant)

GitHub Issue for support in spec: #209
GitHub Issue for support in authz_proxy example: #231
Jira ticket: https://jira.dev.bbc.co.uk/browse/CLOUDFIT-5592

Related PRs

• Implements spec/app note changes made in Add fine-grained auth support to Storage Backends #229

Submitter PR Checks

• PR completes task/fixes bug
• API version has been incremented if necessary
• ADR status has been updated, and ADR implementation has been recorded
• Documentation updated (README, etc.)
• PR added to Jira Issue (if relevant)
• Follow-up stories added to Jira

Reviewer PR Checks

• PR completes task/fixes bug
• Design makes sense, and fits with our current code base
• Code is easy to follow
• PR size is sensible
• Commit history is sensible and tidy

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