Return a valid bearer access token for use at now (Unix seconds),
refreshing via the refresh-token grant first when the current token has
expired (or is within the skew window). The refreshed token is persisted
through the TokenStore. Throws when no valid token can be produced
(e.g. expired with no refresh token).
Return a valid bearer access token for use at now (Unix seconds), refreshing via the refresh-token grant first when the current token has expired (or is within the skew window). The refreshed token is persisted through the TokenStore. Throws when no valid token can be produced (e.g. expired with no refresh token).