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/exec.go

@@ -0,0 +1,44 @@
+package commands
+
+import (
+	"context"
+	"fmt"
+	"io"
+
+	controllerapi "github.com/docker/buildx/controller/pb"
+	"github.com/docker/buildx/monitor/types"
+	"github.com/pkg/errors"
+)
+
+type ExecCmd struct {
+	m types.Monitor
+
+	invokeConfig controllerapi.InvokeConfig
+	stdout       io.WriteCloser
+}
+
+func NewExecCmd(m types.Monitor, invokeConfig controllerapi.InvokeConfig, stdout io.WriteCloser) types.Command {
+	return &ExecCmd{m, invokeConfig, stdout}
+}
+
+func (cm *ExecCmd) Info() types.CommandInfo {
+	return types.CommandInfo{HelpMessage: "execute a process in the interactive container"}
+}
+
+func (cm *ExecCmd) Exec(ctx context.Context, args []string) error {
+	if len(args) < 2 {
+		return errors.Errorf("command must be passed")
+	}
+	cfg := controllerapi.InvokeConfig{
+		Entrypoint: []string{args[1]},
+		Cmd:        args[2:],
+		// TODO: support other options as well via flags
+		Env:  cm.invokeConfig.Env,
+		User: cm.invokeConfig.User,
+		Cwd:  cm.invokeConfig.Cwd,
+		Tty:  true,
+	}
+	pid := cm.m.Exec(ctx, cfg)
+	fmt.Fprintf(cm.stdout, "Process %q started. Press Ctrl-a-c to switch to that process.\n", pid)
+	return nil
+}