[Feature Request] Future proof ClientAssertionProvider

[bgavrilMS]

ClientAssertionProviderBase.GetSignedAssertion callback will not be able to perform CAE (inject claims) or request MTLS POP tokens in its current form.

Updating to use an MSAL overload which has more details. This is a (small) breaking change and should go with Id.Web 3.x

Note: as discussed with @jmprieur, we will probably not move ClientAssertionProvider* classes to MSAL, because we don't want to add Workload federation logic in MSAL.