track_total_hits in request for elastic >= 7.0

[orenov]

Adding track_total_hits param in a request. As it's the only way how to get exact counts using the Search function. https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-track-total-hits.html

Unfortunately, I didn't test it much, but I hope it's ok as it's simple.

I had an assumption that this parameter was introduced with elasticsearch 7.0.

[sckott]

thanks for the PR

• add a track_total_hits parameter to Search_uri function
• please add an argument definition in this file https://github.com/ropensci/elastic/blob/master/man-roxygen/search_par.r for both Search and Search_uri methods