Add support for email address username ("Local Part") for LDAP-based logins

Is your feature request related to a problem? Please describe.

See https://github.com/nextcloud/mail/issues/970 for background - although that issue is not directly related; this idea came from the discussion there-in.

My understanding is that some LDAP implementations do not return a UID which is suitable for logging into the relevant mail server (eg it may be a numeric internal ID) and the email address may not be suitable either as is the case for me.
In my case the "username" part of the email address is required for the mail server login and this username can always be taken from the email address.

Describe the solution you'd like

This feature request is for a feature discussed on issue #970 and which I and others have been using by manually patching our installs for a long time now. Please add the feature so that I can avoid breaking my mail app every time I update it.

Patch file attached
Uploading nextcloud-mail-localpart-ldap-login.txt…

In addition to the above, the string "* %USERID% and %EMAIL% will be replaced with the user's UID and email" below the save config button should be revised to something like:

• %USERID% and %EMAIL% will be replaced with the user's UID and email. %LOCALPART will be replace with the user' email username (the part before the @).

However I don't know how the translations system works.

Describe alternatives you've considered

No response

Additional context

No response

Start by reviewing the background discussion in issue #970 and the attached patch mentioned in this issue. Trace the LDAP-based login configuration and the save-config help text, then check how translations are handled. Done means the requested local-part username option is supported and the configuration text documents its placeholder accurately.