external context should get password from credential store

An external-context has to specify the LDAP passwort inline, either in plain text or as variable via substitution.

It should be possible to get the password via credential-reference from a credential store.

It should work as in a <dir-context><credential-reference store="mystore" .. >

Example for plain text password:

https://docs.wildfly.org/25/Admin_Guide.html#Naming