Potential KeyValueProcessor feature: recursive

[dapowers87]

A boolean specifying whether to drill down into values and recursively get more key-value pairs from it. The extra key-value pairs will be stored as subkeys of the root key.

Default is not to recursive values.

See: https://www.elastic.co/guide/en/logstash/current/plugins-filters-kv.html#plugins-filters-kv-recursive