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

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

docs/src/js/interfaces/ClientConfig.md

@@ -53,3 +53,18 @@ optional tlsConfig: TlsConfig;
 ```ts
 optional userAgent: string;
 ```
+
+***
+
+### userId?
+
+```ts
+optional userId: string;
+```
+
+User identifier for tracking purposes.
+
+This is sent as the `x-lancedb-user-id` header in requests to LanceDB Cloud/Enterprise.
+It can be set directly, or via the `LANCEDB_USER_ID` environment variable.
+Alternatively, set `LANCEDB_USER_ID_ENV_KEY` to specify another environment
+variable that contains the user ID value.