[REFACTOR]: validate custom fields when creating or updating settings

[frascuchon]

Description

This PR adds template validation for custom fields. The validation is a general step when validating settings (settings.validate). The settings.validate method is called before creating or updating the settings, which adds more flexibility to the validations we want to include.

Type of change

• Refactor (change restructuring the codebase without changing functionality)

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[burtenshaw]

Looks good enough to cover basic string/non-string errors.