[GH-9639] Add plugin API for SearchUsers method (#9793)

* fixes 9639

* Adding conversion.

app/plugin_api.go

@@ -316,6 +316,15 @@ func (api *PluginAPI) SearchChannels(teamId string, term string) ([]*model.Chann
 	return *channels, err
 }
 
+func (api *PluginAPI) SearchUsers(search *model.UserSearch) ([]*model.User, *model.AppError) {
+	pluginSearchUsersOptions := &model.UserSearchOptions{
+		IsAdmin:       true,
+		AllowInactive: search.AllowInactive,
+		Limit:         search.Limit,
+	}
+	return api.app.SearchUsers(search, pluginSearchUsersOptions)
+}
+
 func (api *PluginAPI) AddChannelMember(channelId, userId string) (*model.ChannelMember, *model.AppError) {
 	// For now, don't allow overriding these via the plugin API.
 	userRequestorId := ""