Add reusable PR contributor terms workflow with team exemption

[jeffcarp]

Description

Adds a reusable GitHub Actions workflow (.github/workflows/pr-contributor-terms.yml) to verify contributor agreement terms on pull requests across Keras repositories.

Key Features

• Team Exemption: Exempts maintainers and organization members (OWNER, MEMBER, COLLABORATOR) from requiring checked contributor agreement terms.
• Bot Exemption: Automatically exempts bot accounts (e.g. dependabot[bot], google-oss-robot, Renovate).
• Customizable: Allows configurable exempt_associations, exempt_users, and required_terms.
• Informative Failure: Clearly reports any missing/unchecked terms when external contributors submit a PR without checking the boxes.