feat: add user_id field to ClientConfig for user identification (#3240)

## Summary

- Add a `user_id` field to `ClientConfig` that allows users to identify
themselves to LanceDB Cloud/Enterprise
- The user_id is sent as the `x-lancedb-user-id` HTTP header in all
requests
- Supports three configuration methods:
  - Direct assignment via `ClientConfig.user_id`
  - Environment variable `LANCEDB_USER_ID`
  - Indirect env var lookup via `LANCEDB_USER_ID_ENV_KEY`

Closes #3230

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

python/src/connection.rs

@@ -547,6 +547,7 @@ pub struct PyClientConfig {
     id_delimiter: Option<String>,
     tls_config: Option<PyClientTlsConfig>,
     header_provider: Option<Py<PyAny>>,
+    user_id: Option<String>,
 }
 
 #[derive(FromPyObject)]
@@ -631,6 +632,7 @@ impl From<PyClientConfig> for lancedb::remote::ClientConfig {
             id_delimiter: value.id_delimiter,
             tls_config: value.tls_config.map(Into::into),
             header_provider,
+            user_id: value.user_id,
         }
     }
 }