support customSchemaFiles and extraSchemaFiles options

[wkloucek]

What this PR does / why we need it:

The current customSchemaFiles option does a schema import via LDAP_EXTRA_SCHEMAS.
I renamed this current behavior to extraSchemaFiles. It's done via ldapadd while slapd is running.

I added a new customSchemaFiles option that imports schemas via LDAP_CUSTOM_SCHEMA_DIR.
It's done via slapadd while slapd is stopped.

Since the new customSchemaFiles option does not support all schemas that could be set via the current customSchemaFiles option, this change is not backwards compatible. The migration should be easy though because one just needs to use the extraSchemaFiles option instead.

Related issues:

• I wanted to fix Import order of customSchemaFiles not deterministic #104, but the core problem of it was actually fixed in feat(helpers): sort schema apply sequence #161
• duplicates Fixing customSchemaFiles #129

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you updated the readme?
• Is this PR backward compatible? If it is not backward compatible, please discuss open a ticket first

[wkloucek]

@jp-gouin After digging into this PR, I think we don't really benefit from another schema import besides the one we currently have, especially since the import order was fixed in #161. If you have a reasoning for supporting both mechanisms, I'm happy to continue this PR. Otherwise I'll close it.