Activiy sensor/led

**Describe the problem you have/What new integration you would like**

Add an activiy sensor that monitors activity of selectabe components. I.e. wifi, api, uart etc.

**Please describe your use case for this integration and alternatives you've tried:**

Several devices have LEDs and one usage is the status_led, but when having more LEDs available one neat alternative would be to have one showing activity of the device by some flashing pattern or flash by each package sent/received.

Have not found any configuration alternative to achieve this by existing integrations unless exposing new interfaces from them.

**Additional context**

No files or tests are named. Start by reviewing the existing status_led configuration and how Wi-Fi, API, and UART components expose activity. Define how components are selected and how LED flashes represent activity; done means the requested activity sensor can drive an available LED with the described behavior.

Written by the indexing model from the issue text.