Add support for resolving special-use folders

[stevebauman]

IMAP servers can identify standard folders such as Sent, Drafts, Trash, and Archive using RFC 6154 special-use attributes. ImapEngine currently requires consumers to inspect raw mailbox attributes and resolve these folders themselves.

This adds a typed folder data builder that allows consumers to explicitly request supported LIST return data:

$folders = $mailbox->folders()
    ->with(FolderData::SpecialUse)
    ->get();

$sent = $folders->findBySpecialUse(ImapSpecialUse::Sent);

Folder data items share a FolderDataItem contract so parameterized return options can use the same with() API as support for additional extended LIST data is added. Requested capabilities are validated before the command is sent.

Folder mailbox flags have also been renamed to attributes throughout the public API to match IMAP terminology and distinguish them from message flags. FolderCollection resolves the typed ImapSpecialUse enum directly from these attributes, keeping special-use methods off the folder interface. Attributes included voluntarily in ordinary LIST responses are also recognized. Resolution relies exclusively on RFC attributes and returns null instead of guessing from language-dependent folder names. The fake folder repository supports the same builder API for application tests.