Refactor MSIToken

[billwert]

There's a lot of confusing things about MSIToken that came up when I redid its serialization. Here's what a serialized instance looks like:

{"access_token":"token","expires_on":"01/10/2025 15:03:28 +00:00","expires_in":"9600","token":"token","expiresAt":"2025-01-10T15:03:28Z","refreshAt":"2024-11-03T18:17:48.556592450Z"}

Of note, we store the token twice and we have three different date formats. I don't really understand why at this point but we should figure this out.

This is an implementation class so we should be able to rethink this without too much of a compat fear.