MM-18816 Adding ability to add users as another user to the plugin API. (#12562)

* Adding ability to add users as anouther user to the plugin API.

* Documentation feedback.

app/plugin_api.go

@@ -406,6 +406,17 @@ func (api *PluginAPI) AddChannelMember(channelId, userId string) (*model.Channel
 	return api.app.AddChannelMember(userId, channel, userRequestorId, postRootId)
 }
 
+func (api *PluginAPI) AddUserToChannel(channelId, userId, asUserId string) (*model.ChannelMember, *model.AppError) {
+	postRootId := ""
+
+	channel, err := api.GetChannel(channelId)
+	if err != nil {
+		return nil, err
+	}
+
+	return api.app.AddChannelMember(userId, channel, asUserId, postRootId)
+}
+
 func (api *PluginAPI) GetChannelMember(channelId, userId string) (*model.ChannelMember, *model.AppError) {
 	return api.app.GetChannelMember(channelId, userId)
 }

app/plugin_api_test.go

@@ -1439,3 +1439,15 @@ func TestPluginAPIGetUnsanitizedConfig(t *testing.T) {
 		assert.NotEqual(t, config.SqlSettings.DataSourceSearchReplicas[i], model.FAKE_SETTING)
 	}
 }
+
+func TestPluginAddUserToChannel(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	api := th.SetupPluginAPI()
+
+	member, err := api.AddUserToChannel(th.BasicChannel.Id, th.BasicUser.Id, th.BasicUser2.Id)
+	require.Nil(t, err)
+	require.NotNil(t, member)
+	require.Equal(t, th.BasicChannel.Id, member.ChannelId)
+	require.Equal(t, th.BasicUser.Id, member.UserId)
+}