FINERACT-405: Centralize mobile number validation regex

[anubhavio]

Description

Describe the changes made and why they were made. (Ignore if these details are present on the associated Apache Fineract JIRA ticket.)

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[IOhacker]

This should be configurable in a property, not fixed.

[anubhavio]

@IOhacker Thanks for the feedback. I investigated the existing validation and configuration patterns in Fineract. To make the regex configurable, I plan to move it into FineractProperties and replace the Staff @pattern validation with a custom validator backed by a shared PhoneNumberValidationService, so both Client and Staff validations use the same configurable rule.

[IOhacker]

@anubhavio that is the best approach to use the same configurable rule.