feat(container_registry): add size option and size, delete_api_path fields to registry repository

Add the size option to GetSingleRegistryRepositoryOptions.

• Add Size and DeleteAPIPath to RegistryRepository.
• Size is returned only when size=true. DeleteAPIPath is returned only to users who can administer container images, so both can be zero or empty.
• Test: single-repository request with size=true plus response decoding.

Docs: https://docs.gitlab.com/api/container_registry/#get-details-of-a-single-repository