Expose public/ API as submodule (#23345)

* model -> public/model

* plugin -> public/plugin

* public/model/utils -> public/utils

* platform/shared/mlog -> public/shared/mlog

* platform/shared/i18n -> public/shared/i18n

* platform/shared/markdown -> public/shared/markdown

* platform/services/timezones -> public/shared/timezones

* channels/einterfaces -> einterfaces

* expose public/ submodule

* go mod tidy

* .github: cache-dependency-path, setup-go-work

* modules-tidy for public/ too

* remove old gomodtidy

server/public/plugin/doc.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+// The plugin package is used by Mattermost server plugins written in go. It also enables the
+// Mattermost server to manage and interact with the running plugin environment.
+//
+// Note that this package exports a large number of types prefixed with Z_. These are public only
+// to allow their use with Hashicorp's go-plugin (and net/rpc). Do not use these directly.
+package plugin