[feat] Allow Pulsar Proxy-to-Pulsar Proxy communication before reaching broker

### Search before reporting

- [x] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

In some deployment scenarios, it's useful for one proxy to connect through another proxy before reaching the broker. The first proxy may not necessarily be a standard Pulsar Proxy, but as long as it follows the Pulsar protocol rules, the downstream proxy should be able to communicate with it seamlessly.
Supporting this “proxy-to-proxy” connection improves flexibility for complex network topologies, including multi-region deployments, edge-to-central routing, or hierarchical proxy layers. It ensures that authentication, routing, and protocol rules continue to work correctly, even when traffic passes through multiple proxies before reaching the broker.

### Solution

_No response_

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!