Add an aggregator for IPv4 and IPv6 subnets (#82410)

Parameters accepted by the aggregator include:

* prefix_length (integer, required): defines the network size of the subnet mask;
* is_ipv6 (boolean, optional, default: false): defines whether the prefix applies to IPv6 (true) or IPv4 (false) IP addresses;
* min_doc_count (integer, optional, default: 1): defines the minimum number of documents for a bucket to be returned in the results;
* append_prefix_length (boolean, optional, default: false): defines if the prefix length is appended to the IP address key when returning results;
* keyed (boolean, optional, default: false): defines whether the result is returned keyed or as an array of buckets;

Each bucket returned by the aggregator represents a different subnet. IPv4 subnets also include a netmask field set to the subnet mask value (i.e. "255.255.0.0" for a /16 subnet).

Related to: #57964 and elastic/kibana#68424

docs/changelog/82410.yaml

@@ -0,0 +1,5 @@
+pr: 82410
+summary: Add an aggregator for IPv4 and IPv6 subnets
+area: Aggregations
+type: enhancement
+issues: []

docs/reference/aggregations/bucket.asciidoc

@@ -48,6 +48,8 @@ include::bucket/global-aggregation.asciidoc[]
 
 include::bucket/histogram-aggregation.asciidoc[]
 
+include::bucket/ipprefix-aggregation.asciidoc[]
+
 include::bucket/iprange-aggregation.asciidoc[]
 
 include::bucket/missing-aggregation.asciidoc[]