In order to support working with an api key as an authentication method, we need to allow initializing the SDK with an api key that will be used for the base44 api calls fo the client.

• initializing the client should be with either a token or an api key, but not both. reflect that in the types.
• Add tests to ensure the api key is sent as the 'api_key' header.
• Actions like .me() that rely on a specific user token should through an informative error when used with an api key.

@claude do your thing