Instead of hardcoding 'https' in the publicUrl, introduce a scheme variable
that can be configured. This follows the pattern used by other roles
(e.g., matrix_mautrix_discord_scheme, matrix_hookshot_public_scheme).

New variables:
- matrix_appservice_irc_ircService_mediaProxy_publicUrl_scheme (defaults to https)
- matrix_appservice_irc_ircService_mediaProxy_publicUrl (combines scheme, hostname, pathPrefix)

The scheme is wired in group_vars/matrix_servers based on matrix_playbook_ssl_enabled,
consistent with how other roles handle this.