feat: support user-provided aiohttp.ClientSession and configurable proxy host in create_proxy_app

[gspeter-max]

Problem

In src/_bentoml_impl/server/proxy.py (line 67), there is a # TODO: support aiohttp client comment. When a user provides their own aiohttp.ClientSession on their service class via self.client, BentoML detects it but cannot use it correctly.

The root cause: BentoML builds request URLs as relative paths (e.g. /predict) and relies on base_url being set on the ClientSession. BentoML's own client sets base_url=http://localhost:{proxy_port}, so it works. But the user's client has no base_url — they cannot know BentoML's internal proxy_url at the time they create their client.

Additionally, the current proxy hardcodes localhost as the proxy host, which means it cannot forward requests to a remote or external ML server running on a different machine or network.

Use Cases This Enables

1. Custom auth/SSL: User brings a pre-configured client with custom headers, SSL certificates, or retry policies.
2. Remote ML server: User's custom ML server runs on a different machine or container (e.g., http://192.168.1.50:8000), not on localhost.

Current Behavior

# proxy.py line 61-68
proxy_port = service.config.get("http", {}).get("proxy_port", 8000)
proxy_url = f"http://localhost:{proxy_port}"  # hardcoded localhost

if instance_client is not None and isinstance(instance_client, aiohttp.ClientSession):
    # TODO: support aiohttp client
    client = instance_client  # used as-is — breaks because no base_url

Later in reverse_proxy:

url = yarl.URL.build(path=f"/{path}")  # relative path only — no host
client.request(url=url)                 # crashes for user-provided client

Proposed Fix

1. Allow configurable proxy host in service config:

proxy_host = service.config.get("http", {}).get("proxy_host", "localhost")
proxy_port = service.config.get("http", {}).get("proxy_port", 8000)
proxy_url = f"http://{proxy_host}:{proxy_port}"

2. Always build the full absolute URL in reverse_proxy, instead of relying on base_url:

# In reverse_proxy — always construct full URL
full_url = f"{proxy_url.rstrip('/')}/{path}"
client.request(url=full_url, ...)

3. Accept user's client but override its base_url awareness by always passing full URLs.

Files Affected

• src/_bentoml_impl/server/proxy.py

Notes

• When proxy_host is a remote host, should_start_process must be False — BentoML cannot spawn a process on a remote machine.
• This allows users to bring pre-configured clients (auth, SSL, retries) while BentoML handles URL construction.

[gspeter-max]

I am ready to submit a pull request on this issue