No support for default way of authenticating with username and password when reading email.

Currently username and password only works if "Allow public client flows" is set to yes in App registeration in Entra ID. https://github.com/FrendsPlatform/Frends.Exchange/blob/c57dc188b2e161cfd241a0e12bd20805652a21cc/Frends.Exchange.ReadEmail/Frends.Exchange.ReadEmail/Frends.Exchange.ReadEmail.cs#L130 On default it is "no", and as Frends is server application we shouldn't require public flows. If it is "no" also client secret is required on authentication. We made similar change on community side so implementing this here very staright forward as both tasks uses same libraries. https://github.com/CommunityHiQ/Frends.Community.Email/pull/69/files