Add EcoWitt WN20 rain gauge

tests/EcoWitt-WN20/README.md

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+# EcoWitt WN20 Rain Gauge
+
+High-sensitivity tipping bucket rain gauge, 0.3 mm per tip.
+Battery-powered (2xAAA), transmits every 48 seconds at 915 MHz FSK PCM.
+
+Manufacturer page: https://www.ecowitt.com/shop/goodsDetail/325
+
+Captured with `rtl_433 -f 915M -S unknown -T 120` on RTL2832U.
+
+All three captures contain the same device (ID 11399) with 1.9 mm cumulative rain
+and 3.04V battery (fresh 2xAAA). The gateway (GW1100B_V2.4.4) confirms the battery
+voltage matches and reports 100% battery level.
+
+Protocol is identical to the WH40 family (type code 0x40) in ambientweather_wh31e.c,
+but uses type code 0x20, a 10-byte payload (one extra byte before CRC/SUM vs WH40's 9),
+and full-byte battery encoding at 0.02V per unit instead of 5-bit at 0.1V.

tests/EcoWitt-WN20/g025_915M_1000k.json

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+{"time" : "@0.045762s", "model" : "EcoWitt-WN20", "id" : 11399, "battery_V" : 3.040, "battery_ok" : 1.000, "rain_mm" : 1.900, "data" : "0000000000", "mic" : "CRC"}

tests/EcoWitt-WN20/g057_915M_1000k.json

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+{"time" : "@0.072590s", "model" : "EcoWitt-WN20", "id" : 11399, "battery_V" : 3.040, "battery_ok" : 1.000, "rain_mm" : 1.900, "data" : "0000000000", "mic" : "CRC"}
+{"time" : "@0.106322s", "model" : "EcoWitt-WN20", "id" : 11399, "battery_V" : 3.040, "battery_ok" : 1.000, "rain_mm" : 1.900, "data" : "0000000000", "mic" : "CRC"}

tests/EcoWitt-WN20/g093_915M_1000k.json

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+{"time" : "@0.040787s", "model" : "EcoWitt-WN20", "id" : 11399, "battery_V" : 3.040, "battery_ok" : 1.000, "rain_mm" : 1.900, "data" : "0000000000", "mic" : "CRC"}