api, client: don't use a pointer-slice for plugins

[thaJeztah]

The backend and router don't use a pointer-slice (the server actually doesn't use the plugin.ListResponse type and a straight slice);

moby/daemon/server/router/plugin/backend.go

Line 20 in 778e5bf

List(filters.Args) ([]plugintypes.Plugin, error)

moby/daemon/server/router/plugin/plugin_routes.go

Lines 276 to 280 in 6baf274

	l, err := pr.backend.List(pluginFilters)
	if err != nil {
	return err
	}
	return httputils.WriteJSON(w, http.StatusOK, l)

Align the type in the API to match, and update the type defined in the client accordingly.

- What I did

- How I did it

- How to verify it

- Human readable description for the release notes

api/types/plugin: change ListResponse to a non-pointer slice
client: PluginListResult: change Items field to a non-pointer slice

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Perhaps it's time to remove that test;

=== FAIL: amd64.docker.docker.integration.build TestBuildWithHugeFile (35.88s)
    build_test.go:515: assertion failed: string "{\"stream\":\"Step 1/2 : FROM busybox\"}\r\n{\"stream\":\"\\n\"}\r\n{\"stream\":\" ---\\u003e 1c35c4412082\\n\"}\r\n{\"stream\":\"Step 2/2 : RUN for g in $(seq 0 8); do dd if=/dev/urandom of=rnd bs=1K count=1 seek=$((1024*1024*g)) status=none; done  \\u0026\\u0026 ls -la rnd \\u0026\\u0026 du -sk rnd\"}\r\n{\"stream\":\"\\n\"}\r\n{\"stream\":\" ---\\u003e Running in c7d3aea059c1\\n\"}\r\n{\"stream\":\"-rw-r--r--    1 root     root     8589935616 Nov  7 19:05 rnd\\n\"}\r\n{\"stream\":\"36\\trnd\\n\"}\r\n{\"stream\":\" ---\\u003e Removed intermediate container c7d3aea059c1\\n\"}\r\n{\"error\":\"write /rnd: no space left on device\",\"errorDetail\":{\"message\":\"write /rnd: no space left on device\"}}\r\n" does not contain "Successfully built"

no space left on device -> does not contain "Successfully built"