Elasticsearch 7 rest client

[purplesword]

No description provided.

[joschi]

@purplesword Thanks for your contribution!

Could you please resolve the conflicts with the master branch and remove Gradle from the PR? We're not planning to change the build system from Maven to Gradle.

[coveralls]

Coverage increased (+8.6%) to 87.923% when pulling 4ce4c2e on purplesword:elasticsearch_7_rest_client into e2088de on dropwizard:master.

[joschi]

There are quite few settings which might be interesting for the Low Level REST client.
Could we add them here?

https://www.elastic.co/guide/en/elasticsearch/client/java-rest/7.1/java-rest-low-usage-initialization.html
https://www.elastic.co/guide/en/elasticsearch/client/java-rest/7.1/java-rest-low-config.html

Also adding the possibility to add the Sniffer would be nice:
https://www.elastic.co/guide/en/elasticsearch/client/java-rest/7.1/_usage.html

[purplesword]

added a bunch more.. Not sure if did it properly.. Tests will be added soon.

[purplesword]

I added more tests now the coverage is higher and 100% on health checks.

Could you comment more about what do you think is necessary to be covered in configuration?

I added most of them listed in the documentation (but feel a bit hard to write test unless using testcontainer). Also I sort of feel it's a bit too complicate, maybe it's better to cover some simple common configs and then let sophisticated users do what they want with the ManagedClient(client,sniffer) constructor with their own client and sniffer..