SIM800L: Add FM radio support

**Describe the problem you have/What new integration you would like**
SIM800L: has a nice FM radio built-in. Has speaker output. Something like below would be nice:
- switch to turn radio ON/OFF (`AT+FMOPEN` and `AT+FMCLOSE`)
- number to set audio volume (`AT+FMVOLUME`)
- number to set FM frequency (`AT+FMFREQ`)
- action to start FM scanning (`AT+FMSCAN`)
- sensors with currently tuned in frequency and signal strength (`AT+FMSIGNAL`)

Implement it as `media_player`.

**Please describe your use case for this integration and alternatives you've tried:**
The module could be very useful during idle times when no GSM usage is active.

**Additional context**
https://cassiopeia.hk/arduinoradio/
https://cdn-shop.adafruit.com/datasheets/sim800_series_fm_application_note_v1.00.pdf

Start with the linked Arduino radio example and the SIM800 Series FM Application Note to understand the AT+FMOPEN, AT+FMCLOSE, AT+FMVOLUME, AT+FMFREQ, AT+FMSCAN, and AT+FMSIGNAL commands. Done should cover the requested media_player controls plus sensors for tuned frequency and signal strength.

Written by the indexing model from the issue text.