[FIX] microsoft_outlook,google_gmail: require username for OAuth2

Current behavior:

Connections for outgoing email servers using Outlook/Office365 or
Gmail accounts will establish an OAuth2 authentication for the smtp server.
Through the `ir_mail_server` form view, one can fetch the necessary
tokens by logging in into their Microsoft/Gmail account.
Not specifying an username (`smtp_user`) on the `ir_mail_server` record
will not produce an error while fetching those tokens.
But when trying to test the connection or use that server to send an email,
even if the FROM header is correctly set (i.e. the account email address
authorized to sent emails), the smtp connection will fail. This is due
to the fact that when `smtp_user == False`, the respective method
`_generate_outlook_oauth2_string` or respectively `_generate_oauth2_string`
will not be called and send the necessary OAuth2 string when sending
an email through the smtp connection.
This will lead to a `5.7.57 Client not authenticated to send mail.` error.

After this change:

Add specific UserErrors that get called if `smpt_user == False` before
the actions in `open_google_gmail_uri` and `open_microsoft_outlook_uri`
get called. This forces the user to input a `smpt_user` (field Username)
before the login page for OAuth2 gets called to fetch the tokens.
Note: there is no check if the user inputs the right username, only
that the field is not empty. So it is still possible to input an
invalid username.

opw-3268246

closes odoo/odoo#121048

Signed-off-by: Stéphane Debauche (std) <std@odoo.com>