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/kill.go

@@ -0,0 +1,27 @@
+package commands
+
+import (
+	"context"
+
+	"github.com/docker/buildx/monitor/types"
+	"github.com/pkg/errors"
+)
+
+type KillCmd struct {
+	m types.Monitor
+}
+
+func NewKillCmd(m types.Monitor) types.Command {
+	return &KillCmd{m}
+}
+
+func (cm *KillCmd) Info() types.CommandInfo {
+	return types.CommandInfo{HelpMessage: "kill buildx server"}
+}
+
+func (cm *KillCmd) Exec(ctx context.Context, args []string) error {
+	if err := cm.m.Kill(ctx); err != nil {
+		return errors.Errorf("failed to kill: %v", err)
+	}
+	return nil
+}