enable additional/external config location -- outside of install/web-root

[pgnd]

For NC installs, configs are in

	server/config/config.php

That's INSIDE server webroot.

On install, it's populated with secure credentials -- including 'secret', 'passwordsalt', 'dbuser' & 'dbpassword'.

Yes, config files can be excluded from webserver view & ignored by git.

That's subject to 'unfortunate mistakes'.

and, yes, injecting ENV vars is available

Support for config files outside the installation
#7976

Override config.php values through environment variables
#3966

but having params floating around in ENV is not uniquely ideal; e.g. discussion at

https://techbeacon.com/devops/how-keep-your-container-secrets-secure

It'd be useful, at least, to be able to specify an additional config dir outside of webroot.

Perhaps (just) as an option to a cmd-line install, e.g.

	php occ maintenance:install \
	...
	--data-dir="/absolute/path1/to/data/" \
+	--additional-config-dir="/absolute/path2/to/additional-configs/"

Ideally, for config keys/values defined in that additional-config dir, they'd be used by NC -- both at install-time, if preexisting, as well as runtime.

So, e.g, if

/absolute/path2/to/additional-configs/secure.config.php
	$CONFIG = array (
	  'passwordsalt' => 'A...A',
	  'secret'       => 'B...B',
	  'dbpassword'   => 'C...C',
	);

NC would use/write those values ONLY in that file, and not populate them in (webroot)server/config/*