Expose username field for Microsoft oauth2 setup

[Xon]

When using Microsoft oauth2 for bounce/unsubscribe handling and an office 365 account; it is possible to use a single login but to access a shared mailbox by simply filling in the username field.

This allows a cost saving by using a single licensed account ($/m) to access an arbitrary number of unlicensed shared mail boxes instead of needing a license for every login.

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