schemas: legacy: extract ExternalSensor

This allows to remove some properties from 'Aggregate' schema which are
no longer needed, since there is now a more specific schema to use.

Tested: Schema Validator Pass

Change-Id: I31b8a6929ede132c10030a3e416758993e50806a
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/schemas/global.json b/schemas/global.json
index ee1c003..c69b5a4 100644
--- a/schemas/global.json
+++ b/schemas/global.json
@@ -85,6 +85,9 @@
                             "$ref": "legacy.json#/$defs/HostCpuUtilization"
                         },
                         {
+                            "$ref": "legacy.json#/$defs/ExternalSensor"
+                        },
+                        {
                             "$ref": "virtual_sensor.json#/$defs/VirtualSensor"
                         },
                         {