Added custom transport client settings to test infra

It's now possible to define the additional customesettings for transport clients by extending `transportClientSettings` callback method on `ElasticsearchIntegrationTest`.
Aggregations: key_as_string only shown when format specified in terms agg

The key_as_string field is now not shown in the terms aggregation for long and double fields unless the format parameter is specified

Closes #7125
[TEST} make sure unicast bw comp test uses specifically set transport port and properly configure unicast hosts

Added also assertBusy block needed since we test the local cluster state on each node, not only on the master node.
Query Cache: Support shard level query response caching