* Add Shelly emulation for ESP32 (EMUL_SHELLY)
Presents Tasmota as a generation 2 Shelly Pro 2PM to controllers
speaking the Shelly Gen2 RPC protocol, such as the Sigenergy
mySigen application.
The emulation follows Belkin WeMo and Hue Bridge: compiled in
with USE_EMULATION_SHELLY (ESP32 only, as it needs the IDF mDNS
component) and selected at runtime with `Emulation 3` or on the
Configure Other web page, where it appears as "Shelly single
device".
When selected, the driver advertises _shelly._tcp over mDNS with
the IPv4 and IPv6 addresses of all active interfaces and serves
/shelly and /rpc. Up to two switch, two input and two metering
channels are mapped onto Tasmota relays, switches and the energy
driver. Channels beyond the relays configured keep their state
in NVS so a controller still sees two working outputs.
Shelly controllers discard mDNS replies that do not carry the
RFC 6762 TTL of 255, which Espressif's mDNS component only sets
on multicast, so the driver raises the TTL on the mDNS sockets
after registration.
* Make Shelly emulation opt-in and renumber driver
