Werk #15570: Handle redirect RoutingRules of azure's application gateways

If you're having a http redirect routing rule configured in your azure application gateway, the parsing of the section may have failed with:

pydantic.error_wrappers.ValidationError: 10 validation errors for AppGateway

This is due to the fact, that a redirect routing rule may not have the previously required information (backendAddressPool and backendHttpSettings). Those fields are now not required anymore.

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