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/plugintest/mock/mock.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+// This package provides aliases for the contents of "github.com/stretchr/testify/mock". Because
+// external packages can't import our vendored dependencies, this is necessary for them to be able
+// to fully utilize the plugintest package.
+package mock
+
+import (
+	"github.com/stretchr/testify/mock"
+)
+
+const (
+	Anything = mock.Anything
+)
+
+type Arguments = mock.Arguments
+type AnythingOfTypeArgument = mock.AnythingOfTypeArgument
+type Call = mock.Call
+type Mock = mock.Mock
+type TestingT = mock.TestingT
+
+func AnythingOfType(t string) AnythingOfTypeArgument {
+	return mock.AnythingOfType(t)
+}
+
+func AssertExpectationsForObjects(t TestingT, testObjects ...any) bool {
+	return mock.AssertExpectationsForObjects(t, testObjects...)
+}
+
+func MatchedBy(fn any) any {
+	return mock.MatchedBy(fn)
+}