Implement API for getting geographic subdivisions

[nnpvaan]

Description of the issue/feature this PR addresses

This PR implements a new geographic subdivisions API endpoint for SENAITE JSON API that provides access to geographic data (states/provinces, and counties/districts)

Current behavior before PR

Did not provide a dedicated endpoint for geographic subdivisions. Clients requiring geographic data would need to:

• Use the existing address widget's AJAX endpoint which has limited functionality
• Access geographic information only through the core geo API functions directly

Desired behavior after PR is merged

Have access to a comprehensive geographic subdivisions API with the following capabilities:
API Endpoints:

• GET /senaite/v1/geographic/subdivisions_schema - Returns all subdivisions
• GET /senaite/v1/geographic/subdivisions_schema?country=CODE - Returns subdivisions for specific country
• GET /senaite/v1/geographic/subdivisions_schema?depth=LEVEL - Returns subdivisions at specific depth
• GET /senaite/v1/geographic/subdivisions_schema?country=CODE&depth=LEVEL - Returns filtered subdivisions

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I confirm I have tested the PR thoroughly and coded it according to PEP8
standards.