Add OpenSearch 1.x to 3.x migration Docker Compose stack

Description

Create a Docker Compose development environment for testing the OpenSearch 1.x → 3.x migration path. This stack runs both OpenSearch versions side-by-side with dotCMS configured for dual-write mode (FEATURE_FLAG_OPEN_SEARCH_PHASE=1).

Components

• Docker Compose file — OpenSearch 1.3.x, OpenSearch 3.4.0, Dashboards for both, PostgreSQL, and dotCMS
• Python provisioning script (opensearch.py) — creates internal users, roles, action groups, and role mappings on both clusters
• Init containers with healthcheck gates — dotCMS only starts after both clusters are provisioned
• Standalone script using uv run shebang with PEP 723 inline metadata (no virtualenv setup needed)

Service Map

Acceptance Criteria

• docker compose up -d starts all services without errors
• Both provisioners exit 0 (check docker compose ps)
• OpenSearch 1.x responds to authenticated requests on port 9200
• OpenSearch 3.x responds to authenticated requests on port 9201
• dotCMS starts and creates indices on both clusters
• Re-running docker compose up is idempotent (provisioners skip existing users/roles)
• README documents usage, service ports, and provisioning details

Additional Context

PR: https://github.com/dotCMS/core/pull/35864

Start with the Docker Compose file and opensearch.py, then review PR 35864 for the current migration work. Run docker compose up -d and check docker compose ps, authenticated requests on ports 9200 and 9201, dotCMS index creation, and repeated startup. Done means all acceptance criteria pass and the README documents the stack, ports, and provisioning details.