feat: support configurable proxy host and user-provided aiohttp client

[Incheonkirin]

Summary

create_proxy_app hardcoded localhost as the proxy host and could not use a user-provided aiohttp.ClientSession (set via self.client). Closes #5568.

The user's session has no base_url — they cannot know BentoML's internal proxy_port at client-creation time — so the existing code (which relied on base_url and built relative URLs) broke for user-provided clients. The host was also hardcoded to localhost, so requests could not be forwarded to a remote/external upstream server.

Changes

• Configurable proxy host: add optional http.proxy_host (default localhost) so requests can be forwarded to a remote/external ML server.
• Remote-safe process management: when proxy_host is remote, should_start_process is False — BentoML cannot spawn the upstream command on another machine.
• Absolute URLs: build full absolute URLs in reverse_proxy and the startup health check instead of relying on the session's base_url, so a user-provided client works as-is (custom auth/SSL/retries supported).
• Config registration: register proxy_host in the v2 config schema, the SDK HTTPSchema, and the default configuration.

Testing

• py_compile passes for all changed Python files; default config YAML parses.
• Verified URL construction (local / remote / multi-segment path / query string) and the remote-host classification (localhost/127.0.0.1/::1 → local; others → remote).

Notes

Backward compatible: when proxy_host is unset it defaults to localhost, preserving current behavior. No new dependencies.