aggregator and yaml tests for missing agg (#53214)

Tests for unmapped fields, the missing parameter, scripting, and correct
ValuesSource types in MissingAggregatorTests. Basic yaml tests for the
missing agg

For #42949

Author: andyb-elastic

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/320_missing.yml

@@ -0,0 +1,98 @@
+setup:
+  - do:
+      indices.create:
+        index: test
+        body:
+          settings:
+            number_of_replicas: 0
+          mappings:
+            properties:
+              field1:
+                type: long
+              field2:
+                type: long
+              field3:
+                type: long
+  - do:
+      bulk:
+        refresh: true
+        body:
+          - index:
+              _index: test
+          - field1: 100
+          - index:
+              _index: test
+          - field1: 200
+          - index:
+              _index: test
+          - field1: 300
+            field2: 300
+
+---
+"match all":
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        body:
+          aggs:
+            missing_agg:
+              missing:
+                field: field3
+
+  - match: { hits.total: 3 }
+  - length: { hits.hits: 3 }
+
+  - match: { aggregations.missing_agg.doc_count: 3 }
+
+---
+"match some":
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        body:
+          aggs:
+            missing_agg:
+              missing:
+                field: field2
+
+  - match: { hits.total: 3 }
+  - length: { hits.hits: 3 }
+
+  - match: { aggregations.missing_agg.doc_count: 2 }
+
+---
+"match none":
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        body:
+          aggs:
+            missing_agg:
+              missing:
+                field: field1
+
+  - match: { hits.total: 3 }
+  - length: { hits.hits: 3 }
+
+  - match: { aggregations.missing_agg.doc_count: 0 }
+
+---
+"missing param":
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        body:
+          aggs:
+            missing_agg:
+              missing:
+                field: field3
+                missing: 1
+
+  - match: { hits.total: 3 }
+  - length: { hits.hits: 3 }
+
+  - match: { aggregations.missing_agg.doc_count: 0 }