Added Chat External Connections APIs to retrieve external accounts, users, channels, and direct chats. Added the allow_to_add_external_users field to the Chat Channels API.

New External Connections API endpoints


## Added

- New **Chat External Connections** API endpoints:
    - `GET /v2/zoomchat/external_connections/external_users/{member_id}/channels`
    - `GET /v2/zoomchat/external_connections/internal_users/{user_id}/direct_chats`
    - `GET /v2/zoomchat/external_connections/accounts`
    - `GET /v2/zoomchat/external_connections/accounts/{external_account_ref}/channels`
    - `GET /v2/zoomchat/external_connections/internal_users/{user_id}/channels`
    - `GET /v2/zoomchat/external_connections/external_users/{member_id}/direct_chats`
    - `GET /v2/zoomchat/external_connections/accounts/{external_account_ref}/direct_chats`

## Enhancements

- New `allow_to_add_external_users` request field in the following  **Chat Channels** API:
    - `PATCH /v2/chat/channels/{channelId}`
