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/model/group_member.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import "net/http"
+
+type GroupMember struct {
+	GroupId  string `json:"group_id"`
+	UserId   string `json:"user_id"`
+	CreateAt int64  `json:"create_at"`
+	DeleteAt int64  `json:"delete_at"`
+}
+
+func (gm *GroupMember) IsValid() *AppError {
+	if !IsValidId(gm.GroupId) {
+		return NewAppError("GroupMember.IsValid", "model.group_member.group_id.app_error", nil, "", http.StatusBadRequest)
+	}
+	if !IsValidId(gm.UserId) {
+		return NewAppError("GroupMember.IsValid", "model.group_member.user_id.app_error", nil, "", http.StatusBadRequest)
+	}
+	return nil
+}