Authentication Client does not work

[Evan Fuller]

The CreateToken endpoint uses the same underlying impl request helper function as all other API endpoints in this package.

This implementation is hard-coded to use the Bearer prefix when making requests.

Notion's public docs for this endpoint define that it uses the Basic prefix, where the token is the base64-encoded concatenation of an OAuth app's client ID and secret.

Two possible fixes are:

1. Band-aid fix by using the Basic prefix for this endpoint only, and expect library users to pass in the base64-encoded token.
2. Implement a richer Token type that can support both Bearer and Basic cases, the latter of which could include struct fields for ID + Secret, and implement the base64 encoding in this library.

[Anatoly Nosov]

fixed in #145