[MM-13747] Add ability to list users to plugin API (#10102)

* add GetUsers in plugin api

* change GetUsers(page int, perPage int) ([]*model.User, *model.AppError) to GetUsers(options *model.UserGetOptions) ([]*model.User, *model.AppError)

* Update GetUsers doc

Co-Authored-By: dom3k <szeptweb@gmail.com>

* Update doc in plugin/api.go

Co-Authored-By: dom3k <szeptweb@gmail.com>

* correct database initialization for the test

* Update plugin/api.go

Co-Authored-By: dom3k <szeptweb@gmail.com>

app/plugin_api.go

@@ -179,6 +179,10 @@ func (api *PluginAPI) DeleteUser(userId string) *model.AppError {
 	return err
 }
 
+func (api *PluginAPI) GetUsers(options *model.UserGetOptions) ([]*model.User, *model.AppError) {
+	return api.app.GetUsers(options)
+}
+
 func (api *PluginAPI) GetUser(userId string) (*model.User, *model.AppError) {
 	return api.app.GetUser(userId)
 }