Update RefreshAccessTokenResponse with optional refresh_token (#51751)
Update RefreshAccessTokenResponse with optional refresh_token (#51751)

Refreshing accesstokens with the PKCE flow also returns a new refresh_token in the response (https://developer.spotify.com/documentation/general/guides/authorization-guide/#6-requesting-a-refreshed-access-token)
georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped