Add imageLoad API endpoint

[clue]

https://docs.docker.com/reference/api/docker_remote_api_v1.15/#load-a-tarball-with-a-set-of-images-and-tags-into-docker

This API requires passing a valid TAR stream, so this likely depends on clue/reactphp-tar#2 (see also #15)

Complements #19 (imageGet).

[clue]

See also #17 (related, but different)

[clue]

Depends on #33.