refactor database plugin SDK (#29479)

* prepare for enterprise database plugin SDK development

sdk/database/dbplugin/v5/grpc_client.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/golang/protobuf/ptypes"
+
 	"github.com/hashicorp/vault/sdk/database/dbplugin/v5/proto"
 	"github.com/hashicorp/vault/sdk/helper/pluginutil"
 	"github.com/hashicorp/vault/sdk/logical"
@@ -25,6 +26,7 @@ var (
 )
 
 type gRPCClient struct {
+	entGRPCClient
 	client        proto.DatabaseClient
 	versionClient logical.PluginVersionClient
 	doneCtx       context.Context
@@ -285,20 +287,6 @@ func (c gRPCClient) Type() (string, error) {
 	return typeResp.GetType(), nil
 }
 
-func (c gRPCClient) Close() error {
-	ctx, cancel := getContextWithTimeout(pluginutil.PluginGRPCTimeoutClose)
-	defer cancel()
-
-	_, err := c.client.Close(ctx, &proto.Empty{})
-	if err != nil {
-		if c.doneCtx.Err() != nil {
-			return ErrPluginShutdown
-		}
-		return err
-	}
-	return nil
-}
-
 func getContextWithTimeout(env string) (context.Context, context.CancelFunc) {
 	timeout := 1 // default timeout
 	if envTimeout, err := strconv.Atoi(os.Getenv(env)); err == nil && envTimeout > 0 {