feat: refactor the implementation of baseURL

[Oreoxmt]

Refactor the management of server configuration: use a dictionary to manage the server used when calling TiDB Cloud API, the default value is:

{
    "v1beta": "https://api.tidbcloud.com/api/v1beta/",
    "billing": "https://billing.tidbapi.com/v1beta1/"
}

For internal test, you can use:

api = tidbcloudy.TiDBCloud(
	public_key="", private_key="",
	server_config={"v1beta": "xxx", "billing": "xxx"}})