nextcloud: add ability to include custom php file in InstallStep migration

[SergeyMosin]

This is useful for automating deployments. For example a domain can be added automatically, settings changed, etc...

Usage:

php occ config:app:set snappymail custom_config_file --value="/my/custom/config-example.php"

/my/custom/config-example.php

$oProvider = \RainLoop\Api::Actions()->DomainProvider();
$oDomain = $oProvider->Load('cool.domain');
if (!$oDomain) {
	$oDomain = \RainLoop\Model\Domain::fromArray('cool.domain', [
			"IMAP" => [
				"host" => "imap.cool.domain",
				"port" => 993,
				"type" => \MailSo\Net\Enumerations\ConnectionSecurityType::SSL,
				"shortLogin" => false,
			],
			"SMTP" => [
				"host" => "smtp.cool.domain",
				"port" => 465,
				"type" => \MailSo\Net\Enumerations\ConnectionSecurityType::SSL,
				"timeout" => 60,
				"useAuth" => true,
				"shortLogin" => false,
			],
			"Sieve" => [
				"host" => "imap.cool.domain",
				"port" => 4190,
				"enabled" => false
			],
			"whiteList" => "",
		]
	);
	$oProvider->Save($oDomain);
}

$oConfig = \RainLoop\Api::Config();
$oConfig->Set('webmail','title','My Cool SnappyMail');
$oConfig->Save();