Support require_explicit_values on custom properties

## Problem

GitHub's organization custom-property API returns and accepts the optional `require_explicit_values` boolean, but `CustomProperty` cannot read or send it. Clients therefore cannot preserve or manage whether repositories must set an explicit property value.

GitHub documents the field on the organization custom-property schema and create/update endpoints: https://docs.github.com/en/rest/orgs/custom-properties#create-or-update-a-custom-property-for-an-organization

## Proposed change

Add an optional boolean field to `CustomProperty` and cover JSON decoding plus the batch and single-property request bodies with unit tests.

I plan to submit a focused patch.

Locate the CustomProperty type and its organization custom-property request methods, then inspect the existing unit tests for JSON decoding and batch or single-property request bodies. Add coverage for the optional require_explicit_values field in all three cases, and confirm the tests verify both reading and sending the boolean.

Written by the indexing model from the issue text.