Consider to support API call without access-token

[kogai]

Let me check how to use the Message API to get a short live access token 🙏

According to the documentation,
It isn't necessary Authorization header to issue an access token.

Nevertheless, to instantiate Client for enable to call IssueAccessTokenV2 method requires an access token itself 🤔

Fortunately, in a case of me, thanks to exposing APIEndpointBase and APIEndpointIssueAccessToken,
It was easy to issue an appropriate request by using http module of std-lib.

By the way, If any function or structure which supports some interface that does not require an access token might be nice.

Thanks for reading.

[Yang-33]

#392 supports this!