Complete remote driver

This patch completes the work started in creating a remote driver:

- Renames the env driver to the remote driver (an alternative suggestion
  that should be more user-friendly)
- Adds support for TLS to encrypt connections with buildkitd
- Fixes outstanding review comments
- Reworks the buildx create command endpoint construction to be clearer
  and include better support for this new driver.

Signed-off-by: Justin Chadwell <me@jedevc.com>

driver/remote/driver.go

@@ -0,0 +1,80 @@
+package remote
+
+import (
+	"context"
+
+	"github.com/docker/buildx/driver"
+	"github.com/docker/buildx/util/progress"
+	"github.com/moby/buildkit/client"
+	"github.com/pkg/errors"
+)
+
+type Driver struct {
+	factory driver.Factory
+	driver.InitConfig
+	*tlsOpts
+}
+
+type tlsOpts struct {
+	serverName string
+	caCert     string
+	cert       string
+	key        string
+}
+
+func (d *Driver) Bootstrap(ctx context.Context, l progress.Logger) error {
+	return nil
+}
+
+func (d *Driver) Info(ctx context.Context) (*driver.Info, error) {
+	c, err := d.Client(ctx)
+	if err != nil {
+		return nil, errors.Wrapf(driver.ErrNotConnecting, err.Error())
+	}
+
+	if _, err := c.ListWorkers(ctx); err != nil {
+		return nil, errors.Wrapf(driver.ErrNotConnecting, err.Error())
+	}
+
+	return &driver.Info{
+		Status: driver.Running,
+	}, nil
+}
+
+func (d *Driver) Stop(ctx context.Context, force bool) error {
+	return nil
+}
+
+func (d *Driver) Rm(ctx context.Context, force, rmVolume, rmDaemon bool) error {
+	return nil
+}
+
+func (d *Driver) Client(ctx context.Context) (*client.Client, error) {
+	opts := []client.ClientOpt{}
+	if d.tlsOpts != nil {
+		opts = append(opts, client.WithCredentials(d.tlsOpts.serverName, d.tlsOpts.caCert, d.tlsOpts.cert, d.tlsOpts.key))
+	}
+
+	return client.New(ctx, d.InitConfig.EndpointAddr, opts...)
+}
+
+func (d *Driver) Features() map[driver.Feature]bool {
+	return map[driver.Feature]bool{
+		driver.OCIExporter:    true,
+		driver.DockerExporter: false,
+		driver.CacheExport:    true,
+		driver.MultiPlatform:  true,
+	}
+}
+
+func (d *Driver) Factory() driver.Factory {
+	return d.factory
+}
+
+func (d *Driver) IsMobyDriver() bool {
+	return false
+}
+
+func (d *Driver) Config() driver.InitConfig {
+	return d.InitConfig
+}