Ability to use search templates

[jguay]

Plugin allows for :

• query_template : where query_string is stored on disk as per your example
• [query](https://www.elastic.co/guide/en/logstash/7.4
  /plugins-filters-elasticsearch.html#plugins-filters-elasticsearch-query) : where query_string is passed directly to be executed
• Also use of query_string for nested document is documented to be avoided but can work

Requested Feature :
New setting to be able to use _search/template API with list of available fields from the plugin passed for arguments for elasticsearch search template

This would give the ability to maintain the search centrally