Add API for synchronous token validation

[ByronBecker]

I'm currently storing the generated RTC token in local storage, and would like to be able to verify that the generated token is valid/expired without making an API call to the server.

This way, if the token is invalid I can save a round trip (not needing to receive a 403 before refreshing the token).

Ideally this API would live in the agora-token repository 🙏