monitor: Move commands to a separated package

This commit moves monitor commands to `monior/commands` package.
Commands still need access to the `monitor` object and buildx controller so this
commit enables this via `Monitor` interface stored in `monitor/types`.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

monitor/commands/list.go

@@ -0,0 +1,40 @@
+package commands
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"sort"
+	"text/tabwriter"
+
+	"github.com/docker/buildx/monitor/types"
+)
+
+type ListCmd struct {
+	m types.Monitor
+
+	stdout io.WriteCloser
+}
+
+func NewListCmd(m types.Monitor, stdout io.WriteCloser) types.Command {
+	return &ListCmd{m, stdout}
+}
+
+func (cm *ListCmd) Info() types.CommandInfo {
+	return types.CommandInfo{HelpMessage: "list buildx sessions"}
+}
+
+func (cm *ListCmd) Exec(ctx context.Context, args []string) error {
+	refs, err := cm.m.List(ctx)
+	if err != nil {
+		return err
+	}
+	sort.Strings(refs)
+	tw := tabwriter.NewWriter(cm.stdout, 1, 8, 1, '\t', 0)
+	fmt.Fprintln(tw, "ID\tCURRENT_SESSION")
+	for _, k := range refs {
+		fmt.Fprintf(tw, "%-20s\t%v\n", k, k == cm.m.AttachedSessionID())
+	}
+	tw.Flush()
+	return nil
+}