OAuth2 token exchange.

AccessToken is the token that authorizes and authenticates the requests.

ExpiresIn is the OAuth2 wire format "expires_in" field, which specifies how many seconds later the token expires, relative to an unknown time base approximately around "now". It is the application's responsibility to populate Expiry from ExpiresIn when required.

Expiry is the optional expiration time of the access token.

If zero, [TokenSource] implementations will reuse the same token forever and RefreshToken or equivalent mechanisms for that TokenSource will not be used.

RefreshToken is a token that's used by the application (as opposed to the user) to refresh the access token if it expires.

TokenType is the type of token. The Type method returns either this or "Bearer", the default.