[Feature Request] Simplify updating client assertions

[chlowell]

Because client assertions are baked into Credential instances, which are in turn baked into confidential.Client instances, it's necessary to create a new instance of each when an assertion expires. Assertions will commonly expire, so it would be great to have a more efficient API for rotating them in MSAL. For example, instead of a string parameter, NewCredFromAssertion() could take a callback that returns assertions (something like a func (context.Context) (string, error)).