[FR] IMAP: Allow use of subdomain only as host

[narcisgarcia]

Is your feature request related to a problem? Please describe.
I'm hosting Nextcloud for CardDAV/CalDAV logins from IMAP authentication, but this NC instance needs to host accounts from different mail services.
I can configure:

 'user_backends' => 
 array (
   0 => 
   array (
     'class' => '\\OCA\\UserExternal\\IMAP',
     'arguments' => 
     array (
       0 => 'imap.example.net',
       1 => 993,
       2 => 'ssl',
       3 => ,
       4 => false,
       5 => false,
     ),
   ),
 ),

to accounts@example.net that authenticate to imap.example.net
but I can't configure accounts@example.COM that need to authenticate to imap.example.COM

Describe the solution you'd like
My proposal is to allow specify only subdomain as IMAP server:

 'user_backends' => 
 array (
   0 => 
   array (
     'class' => '\\OCA\\UserExternal\\IMAP',
     'arguments' => 
     array (
       0 => 'imap.',
       1 => 993,
       2 => 'ssl',
       3 => ,
       4 => false,
       5 => false,
     ),
   ),
 ),

This way, when External user authentication extension finds first argument ending with a dot '.' it should append e-mail address FQDN to argument:
On 0 => 'sub.', and username is "john@example.net" concatenate "sub." + "example.net" to get "sub.example.net" as IMAP host.