Add 'domain' field to FeishuConfig (Literal['feishu', 'lark'], default 'feishu').
Pass domain to lark.Client.builder() and lark.ws.Client to support Lark global
(open.larksuite.com) in addition to Feishu China (open.feishu.cn).
Existing configs default to 'feishu' for backward compatibility.
Also add documentation for domain field in README.md and add tests for
domain config.