saveOauthAccessToken
Persists the OAuth2 access token, its expiry, token type, scope, and refresh token.
Both the access token and the refresh token expiry are stored as absolute epoch millisecond values derived from their respective expiresIn fields (seconds).
Parameters
token
The Oauth2AccessToken received from the MTN MoMo OAuth2 token endpoint, or null (in which case this call is a no-op).