accounts/email: add protocol-specific userName options

[MunsMan]

Add optional userName fields to IMAP and SMTP submodules, allowing
different usernames for IMAP and SMTP servers. When not specified,
the account-level userName is used as a fallback.

This change updates all email programs (mbsync, msmtp, offlineimap,
getmail, neomutt, aerc, thunderbird, git) to use the protocol-specific
username when available.

Description

Checklist

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[teto]

could you share an example where it would be useful ? if we introduce complexity, we need to justify it

[MunsMan]

Sure! I'm personally facing this problem with an account provided by my university. The issue is that I can't manage a single account outside of Home-Manager, because every switch reverts my manual changes.