feat(search): avg metric aggregation

Adds `avg` as a permitted value on `aggregationOption.metricKind` and
`searchAggregation.metricKind`. Distinct from sum/min/max because an
average of two averages isn't mergeable: the backend has to carry
(sum, count) internally per bucket and only collapse to the scalar
`value` at the outermost merge.

Wire shape stays symmetrical with sum/min/max: callers still just
read `value` off the response; the (sum, count) bookkeeping is
entirely service-side.

Author: dschmidt

api/openapi-spec/v1.0.yaml

@@ -6579,10 +6579,15 @@ components:
             `searchAggregation` in the response carries a `value`
             rather than `buckets`. Libregraph extension not present in
             MS Graph.
+
+            `avg` is not a simple reducer (averages of averages are
+            not averages): the backend carries `(sum, count)`
+            internally and emits only `value` on the outermost merge.
           enum:
             - sum
             - min
             - max
+            - avg
     bucketDefinition:
       type: object
       description: |
@@ -6769,6 +6774,7 @@ components:
             - sum
             - min
             - max
+            - avg
     searchBucket:
       type: object
       description: |