feat: Add gateway_priority support for network endpoints

[Adithya1331]

This pull request introduces support for the gateway_priority parameter, allowing users to specify the gateway selection priority for a container's network endpoint. This feature corresponds to the GwPriority field in the Docker Engine API, available from version 1.48.

Key Changes:

• docker.types.networks.EndpointConfig:
  • Added gw_priority parameter to the constructor.
  • Includes validation for API version (>= 1.48) and integer type.
• docker.api.network.NetworkApiMixin.connect_container_to_network:
  • Added optional gw_priority parameter, which is passed to EndpointConfig.
• docker.api.container.ContainerApiMixin.create_endpoint_config:
  • Added optional gw_priority parameter, allowing create_container to utilize this feature via networking_config.
• Tests:
  • Added new unit tests (tests/unit/api_network_gw_priority_test.py) for EndpointConfig and the connect_container_to_network method's handling of gw_priority.
  • Updated existing unit tests in tests/unit/api_network_test.py and tests/unit/api_container_test.py.
  • Added new integration tests in tests/integration/api_network_test.py and tests/integration/api_container_test.py to verify functionality against a Docker daemon.
• Documentation:
  • Updated docstrings for all modified methods and classes to include gw_priority.
  • Added an entry to docs/change_log.md.

This implementation allows users to influence which network endpoint provides the default gateway for a container, as per the Docker API v1.48 specification.