AccessToken is the token that authorizes and authenticates
the requests.
expires_ininteger
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.
expirystring
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.
refresh_tokenstring
RefreshToken is a token that's used by the application
(as opposed to the user) to refresh the access token
if it expires.
token_typestring
TokenType is the type of token.
The Type method returns either this or "Bearer", the default.