Add Ubiquiti Flex-XG (#1399)

* Add Ubiquiti Flex-XG

* Update USW-Flex-XG.yaml

Added poe_mode and poe_type

* Update USW-Flex-XG.yaml

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Co-authored-by: Daniel W. Anner <daniel.anner@danstechsupport.com>

device-types/Ubiquiti/USW-Flex-XG.yaml

+---
+manufacturer: Ubiquiti
+model: UniFi Switch Flex XG
+slug: ubiquiti-unifi-switch-flex-xg
+part_number: USW-Flex-XG
+weight: 1.2
+weight_unit: kg
+comments: |
+  135 x 185 x 32 mm (5.31 x 7.28 x 1.27")
+u_height: 0
+is_full_depth: false
+interfaces:
+  - name: Port 1 (PoE IN)
+    type: 1000base-t
+    poe_mode: pd
+    poe_type: type2-ieee802.3at
+  - name: Port 2
+    type: 10gbase-t
+  - name: Port 3
+    type: 10gbase-t
+  - name: Port 4
+    type: 10gbase-t
+  - name: Port 5
+    type: 10gbase-t