feat(helpers): sort schema apply sequence

[Arano-kai]

What this PR does / why we need it:

Custom schema filenames are inserted into LDAP_EXTRA_SCHEMAS in an unsorted fashion, causing inconsistencies every time a new schema is added, or even in a different version of the helm. This commit enable customSchemaFiles keys sorting, allowing control over the loading sequence and using dependent custom schemas.

Pre-submission checklist:

• [x] Did you explain what problem does this PR solve? Or what new features have been added?
• [x] Have you updated the readme? (noted in values.yaml)
• [x] Is this PR backward compatible? If it is not backward compatible, please discuss open a ticket first

[Arano-kai]

Note: this PR conflicts with #129, do not forget to merge if both are accepted.