Update mattermost library

vendor/github.com/hashicorp/go-plugin/constants.go

@@ -0,0 +1,16 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package plugin
+
+const (
+	// EnvUnixSocketDir specifies the directory that _plugins_ should create unix
+	// sockets in. Does not affect client behavior.
+	EnvUnixSocketDir = "PLUGIN_UNIX_SOCKET_DIR"
+
+	// EnvUnixSocketGroup specifies the owning, writable group to set for Unix
+	// sockets created by _plugins_. Does not affect client behavior.
+	EnvUnixSocketGroup = "PLUGIN_UNIX_SOCKET_GROUP"
+
+	envMultiplexGRPC = "PLUGIN_MULTIPLEX_GRPC"
+)