fix!: use client ip when creating connection logs for workspace proxied app accesses (#19788)

Breaking API Change: 
> The presence of the `ip` field on `codersdk.ConnectionLog` cannot be
guaranteed, and so the field has been made optional. It may be omitted
on API responses.

When running a scaletest, I noticed logs of the form:
```
2025-09-12 06:34:10.924 [erro]  coderd.workspaceapps: upsert connection log failed  trace=0xa17580  span=0xa17620  workspace_id=81b937d7-5777-4df5-b5cb-80241f30326f  agent_id=78b2ff6d-b4a6-4a4e-88a7-283e05455a88  app_id=00000000-0000-0000-0000-000000000000  user_id=00000000-0000-0000-0000-000000000000  user_agent=""  app_slug_or_port=terminal  status_code=404  request_id=67f03cf8-9523-444a-97bc-90de080a54c8 ...
    error= 1 error occurred:
           	* pq: null value in column "ip" of relation "connection_logs" violates not-null constraint
```

to ensure logs are never omitted from the connection log due to a
missing IP again (i.e. I'm not sure if we can always rely on a valid,
parseable, IP from `(http.Request).RemoteAddr`), I've removed the `NOT
NULL` constraint on `ip` on `connection_logs`, and made `ip` on the API
response optional.


The specific cause for these null IPs was the
`/workspaceproxies/me/issue-signed-app-token [post]` endpoint
constructing it's own `http.Request` without a `RemoteAddr` set, and
then passing that to the token issuer.

To solve this, we'll have workspace proxies send the real IP of the
client when calling `/workspaceproxies/me/issue-signed-app-token [post]`
via the header `Coder-Workspace-Proxy-Real-IP`.

coderd/database/dump.sql

@@ -891,7 +891,7 @@ CREATE TABLE connection_logs (
     workspace_name text NOT NULL,
     agent_name text NOT NULL,
     type connection_type NOT NULL,
-    ip inet NOT NULL,
+    ip inet,
     code integer,
     user_agent text,
     user_id uuid,