Per-network metadata enable/disable for ML2/OVN

The spec covers:

• A new network-metadata API extension adding the Boolean metadata_enabled attribute to the network resource.
• A new networkmetadata DB table (with Alembic migration).
• Changes to the ML2/OVN mechanism driver to create or delete the metadata port based on the per-network attribute.
• Changes to the OVN agent metadata extension to respect the per-network flag when spawning metadata-proxy processes.
• Upgrade impact: existing networks inherit the global default, so behaviour is unchanged until the attribute is explicitly set.