Add test feature for bfloat16

Author: thecoop

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/41_knn_search_bbq_hnsw_bfloat16.yml

@@ -1,4 +1,7 @@
 setup:
+  - requires:
+      cluster_features: "mapper.bbq_bfloat16"
+      reason: 'bfloat16 needs to be supported'
   - do:
       indices.create:
         index: bbq_hnsw

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/42_knn_search_bbq_flat_bfloat16.yml

@@ -1,4 +1,7 @@
 setup:
+  - requires:
+      cluster_features: "mapper.bbq_bfloat16"
+      reason: 'bfloat16 needs to be supported'
   - do:
       indices.create:
         index: bbq_flat

server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java

@@ -47,6 +47,7 @@ public class MapperFeatures implements FeatureSpecification {
     static final NodeFeature BBQ_DISK_SUPPORT = new NodeFeature("mapper.bbq_disk_support");
     static final NodeFeature SEARCH_LOAD_PER_SHARD = new NodeFeature("mapper.search_load_per_shard");
     static final NodeFeature PATTERNED_TEXT = new NodeFeature("mapper.patterned_text");
+    static final NodeFeature BBQ_BFLOAT16 = new NodeFeature("mapper.bbq_bfloat16");
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
@@ -80,7 +81,8 @@ public Set<NodeFeature> getTestFeatures() {
             BBQ_DISK_SUPPORT,
             SEARCH_LOAD_PER_SHARD,
             SPARSE_VECTOR_INDEX_OPTIONS_FEATURE,
-            PATTERNED_TEXT
+            PATTERNED_TEXT,
+            BBQ_BFLOAT16
         );
     }
 }