Add support for max_depth

[tumluliu]

• Version: v3.1.0 (with logstash 7.4.0)
• Operating System: Linux
• Config File (if you have sensitive info, please remove it):
  as simple as

filter {
      json {
        source => "message"
      }
    }

• Sample Data:
• Steps to Reproduce:

It would be great to have a max_depth option as in Filebeat's decode_json_fields processor. Otherwise, ES would be spammed by the decoded json fields. The current solution from the forum or stackoverflow is too ugly to apply.