Fix non-deterministic fingerprint for nested hashes by sorting

Consider this example:

```JSON
{
  "@timestamp": "2023-05-23T23:23:23.555Z",
  "@Version": "1",
  "beat": {
    "hostname": "gnu.example.com",
    "name": "gnu.example.com",
    "version": "5.2.2"
  },
  "host": "gnu.example.com"
}
```

Using this filter:

```Logstash
fingerprint {
  concatenate_all_fields => true
  target => "[@metadata][_id]"
  method => "SHA512"
  key => "XXX"
  base64encode => true
}
```

Here, the order of the `.beat` hash is non-deterministic and the plugin
did not do a deep sort as part of the serialization. This resulted in
different fingerprints for the same event because the order of the three
keys (hostname, name, version) changed randomly in the serialization.

This has been fixed by recursively checking for hashes and serializing
them in sorted order.

Note that this changes the serialization format and thus breaks
backwards compatibility. The old format could be emulated in order to
not break backwards compatibility. Backwards compatibility in this case
means to generate the same fingerprint for the same input.

Closes: #39

Author: ypid-geberit

lib/logstash/filters/fingerprint.rb

@@ -104,6 +104,21 @@ class << self; alias_method :fingerprint, :fingerprint_openssl; end
     end
   end
 
+  def serialize(event)
+    to_string = ""
+    if event.respond_to?(:to_hash)
+      to_string << "{"
+      event.to_hash.sort.map do |k,v|
+        to_string << "#{k}:#{serialize(v)},"
+      end
+      to_string << "}"
+    else
+      to_string << "#{event}"
+    end
+
+    return to_string
+  end
+
   def filter(event)
     case @method
     when :UUID
@@ -120,12 +135,10 @@ def filter(event)
       if @concatenate_sources || @concatenate_all_fields
         to_string = ""
         if @concatenate_all_fields
-          event.to_hash.sort.map do |k,v|
-            to_string << "|#{k}|#{v}"
-          end
+          to_string << serialize(event)
         else
           @source.sort.each do |k|
-            to_string << "|#{k}|#{event.get(k)}"
+            to_string << "|#{k}|#{serialize(event.get(k))}"
           end
         end
         to_string << "|"