Allow a dynamic string in user password config

[MiChAeLoKGB]

SecurityExtension->getConfigSchema expects password to be a string, instead of a dynamic string.

Because of this, there is no way to use environment variables to do a simple user setup in e.g. k8s.

You could make a custom authenticator, store values in DB etc., but there are apps where you basically have a single admin, and rest of the app is public (so no user auth needed).

In such cases, being able to use following config would be really helpful:

security:
	users:
		admin: '%env.admin_password%'

security:
	users:
		admin: 
			password: '%env.admin_password%'