Add missing host.* fields to fields.yml (elastic#11016)

The `add_host_metadata` processor (default since 6.5) fills the fields `host.os.build` and `host.containerized`, but there is no entry for them in `fields.yml` so they are missing from the generated index patterns.

This adds them to `add_host_metadata/_meta/fields.yml`.

elastic#10992 already addressed this for 6.7.

docs/fields.asciidoc

@@ -3960,6 +3960,30 @@ Hostname of the agent.
 Info collected for the host machine.
 
 
+
+
+*`host.containerized`*::
++
+--
+type: boolean
+
+If the host is a container.
+
+
+--
+
+*`host.os.build`*::
++
+--
+type: keyword
+
+example: 18D109
+
+OS build information.
+
+
+--
+
 [[exported-fields-kubernetes-processor]]
 == Kubernetes fields