plugin/discovery: build plugin clients

These new methods ClientConfig and Client provide the bridge into the
main plugin infrastructure by configuring and instantiating (respectively)
a client object for the referenced plugin.

This stops short of getting the proxy object from the client since that
then requires referencing the interface for the plugin kind, which would
then create a dependency on the main terraform package which we'd rather
avoid here. It'll be the responsibility of the caller in the command
package to do the final wiring to get a provider instance out of a
provider plugin client.

plugin/discovery/meta.go

@@ -4,8 +4,11 @@ import (
 	"crypto/sha256"
 	"io"
 	"os"
+	"os/exec"
 
 	"github.com/blang/semver"
+	plugin "github.com/hashicorp/go-plugin"
+	tfplugin "github.com/hashicorp/terraform/plugin"
 )
 
 // PluginMeta is metadata about a plugin, useful for launching the plugin
@@ -48,3 +51,19 @@ func (m PluginMeta) SHA256() ([]byte, error) {
 
 	return h.Sum(nil), nil
 }
+
+// ClientConfig returns a configuration object that can be used to instantiate
+// a client for the referenced plugin.
+func (m PluginMeta) ClientConfig() *plugin.ClientConfig {
+	return &plugin.ClientConfig{
+		Cmd:             exec.Command(m.Path),
+		HandshakeConfig: tfplugin.Handshake,
+		Managed:         true,
+		Plugins:         tfplugin.PluginMap,
+	}
+}
+
+// Client returns a plugin client for the referenced plugin.
+func (m PluginMeta) Client() *plugin.Client {
+	return plugin.NewClient(m.ClientConfig())
+}