Add App Password functionality to handle 2FA for outgoing emails

[usonianhorizon]

Outlook as part of O365 and Gmail have App Passwords to allow outgoing email via SMTP.

• Enforce Microsoft Entra multifactor authentication with legacy applications using app passwords

• Google: Sign in with app passwords

In order to connect LedgerSMB to these email services App Password capability needs to be added.

[usonianhorizon]

I may have completely misunderstood App Password mechanism. From my re-reading, it would seem that generating an App Password allows one to bypass 2FA for legacy apps, and so my asking for App Password functionality to handle 2FA doesn't make sense.

[ehuelsmann]

Our configuration mechanism allows for configuration of many different mail transports, including hooking up "transactional e-mail services" like Mailgun with the right plugins, including a directly available Mailgun plugin.

For different types of authentication using a regular SMTP service, the included SMTP transport provides an sasl_authenticator attribute which can hold an Authen::SASL instance. I'm the maintainer of that module; if there are any methods missing, we can extend it.