cli/command/system: define struct for formatting version

The client.ServerVersion method in the moby/client module defines
an output struct that's separate from the API response. These output
structs are not designed to be marshaled as JSON, but the CLI depended
on them defining `json` labels, which it used to format the output
as JSON (`docker version --format=json`); as a result, the JSON output
changed in docker v29, as it would now use the naming based on the Go
struct's fields (`APIVersion` instead of `ApiVersion`).

In future, we should consider having a `--raw` (or similar) option for
the CLI to print API responses as-is, instead of using client structs
or CLI structs for this (this would also make sure the JSON output does
not inherit client-side formatting of fields).

For now, let's create a struct for formatting the output, similar to what
we do for the client-side information.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/version_test.go

@@ -47,8 +47,18 @@ func TestVersionFormat(t *testing.T) {
 			BuildTime:         "Wed May 30 22:21:05 2018",
 			Context:           "my-context",
 		},
-		Server: &client.ServerVersionResult{
-			Platform: client.PlatformInfo{Name: "Docker Enterprise Edition (EE) 2.0"},
+		Server: &serverVersion{
+			Platform:      client.PlatformInfo{Name: "Docker Enterprise Edition (EE) 2.0"},
+			Version:       "18.99.5-ce",
+			APIVersion:    "1.30",
+			MinAPIVersion: "1.12",
+			Os:            "linux",
+			Arch:          "amd64",
+			GitCommit:     "64ddfa6",
+			GoVersion:     "go1.8.7",
+			KernelVersion: "v1.0.0",
+			Experimental:  false,
+			BuildTime:     "Mon Jul  9 23:38:38 2018",
 			Components: []system.ComponentVersion{
 				{
 					Name:    "Engine",