Add some group plugin APIs (#12180)

* Add group store GetByUser

* Add group store GetByName

* Add group plugin APIs

* Minor naming fixes

* Add minimum version comments

app/plugin_api.go

@@ -445,6 +445,18 @@ func (api *PluginAPI) DeleteChannelMember(channelId, userId string) *model.AppEr
 	return api.app.LeaveChannel(channelId, userId)
 }
 
+func (api *PluginAPI) GetGroup(groupId string) (*model.Group, *model.AppError) {
+	return api.app.GetGroup(groupId)
+}
+
+func (api *PluginAPI) GetGroupByName(name string) (*model.Group, *model.AppError) {
+	return api.app.GetGroupByName(name)
+}
+
+func (api *PluginAPI) GetGroupsForUser(userId string) ([]*model.Group, *model.AppError) {
+	return api.app.GetGroupsByUserId(userId)
+}
+
 func (api *PluginAPI) CreatePost(post *model.Post) (*model.Post, *model.AppError) {
 	return api.app.CreatePostMissingChannel(post, true)
 }