Add IS access token callback
Add IS access token callback

This passes a callback to the JS SDK which it can use to get IS access tokens
whenever needed for either talking to the IS directly or passing along to the
HS.

Fixes https://github.com/vector-im/riot-web/issues/10525

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk