Add multi-get support

[npm1]

While our current focus is on allowing multiple IDPs to be supported from a single get() call, there are use cases where this is insufficient, so we should also have a path towards allowing multiple independent IDPs to be shown in the FedCM UI of a page.

[npm1]

My hot take: let's solve this with dynamic UI (show accounts as they become available). Maaybe add a method for RP to mention which IDPs it expects as well? Although that method risks adding a lot of complexity (wait until when?)

The other issue with multi-get is that parameters may be mismatched: what happens with different rp contexts? Different mediations? different modes? etc