[Identity] UsernamePasswordCredential API alignment

[sadasant]

Remove authenticationRecord and disableAutomaticAuthentication from the optional parameters to this credential.

While MSAL does support this, allowing authenticationRecord arguably could result in users authenticating through an account other than the one they're specifying with the username and the password.

[sadasant]

Pushed to May because I won't have time to get this reviewed before the April milestone ends.

[sadasant]

@christothes @schaabs should we also drop the "authenticate()" method from this credential? I see it in .Net: https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/identity/Azure.Identity/api/Azure.Identity.netstandard2.0.cs#L264-L265