Backport of ext-authz Envoy extension: support localhost as a valid target URI into release/1.16.0

[cthain]

Backport

This PR is for backporting #17821 to release/1.16.0

The below text is copied from the body of the original PR.

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Description

This PR fixes a bug in the builtin/ext-authz extension when using localhost as the target URI. When using an upstream on the localhost network we configure a STATIC cluster type that requires using an IP address. The extension was configuring the cluster with the localhost hostname, which caused Envoy to reject the xDS configuration update with an error similar to:

2023-06-20T16:27:39.907Z [ERROR] agent.envoy.xds: got error response from envoy proxy: service_id=default/default/api-5f54fbf97-vkpgr-api-sidecar-proxy typeUrl=type.googleapis.com/envoy.config.cluster.v3.Cluster xdsVersion=v3 nonce=00000006 error="rpc error: code = Internal desc = Error adding/updating cluster(s) local_ext_authz: malformed IP address: localhost. Consider setting resolver_name or setting cluster type to 'STRICT_DNS' or 'LOGICAL_DNS'"

Testing & Reproduction steps

• I found this testing the ext_authz extension using Consul on k8s.
  • I tested the fix successfully in this same environment
• I've updated the agent/xds golden tests to ensure that IP addresses result in a STATIC cluster type and that localhost results in a STRICT_DNS cluster type, when creating the cluster definition.
• Added some additional unit tests to the ext-authz extension to cover some additional error cases.

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

[NicoletaPopoviciu]

LGTM compared w the original PR