[GCSS] feat: add Custom Properties support

[tabathad]

Summary

Add support for managing GitHub custom repository properties through GCSS/GitHub Terraformer, enabling repository lifecycle labeling for GR-OSS projects.

Proposed Custom Property: oss-lifecycle

A single-select property with the following allowed values:

State	Description
published	Newly released project. Assigned for the first 90 days after public release.
active	Project is actively developed and maintained. Regular releases and community engagement.
contributing	We contribute to an upstream project we don't own. Tracks participation in external projects.
maintained	Stable project receiving security patches and dependency updates, but no active feature development.
evaluating	Project under review to determine next steps. 90-day review cycle.
deprecated	No longer recommended for use. Not maintained. Should include migration guidance.
archived	Read-only. No further development, patches, or support.

• YAML config example:

custom_properties:
  oss-lifecycle: active

reference:
https://docs.github.com/en/enterprise-cloud@latest/admin/managing-accounts-and-repositories/managing-organizations-in-your-enterprise/managing-custom-properties-for-organizations