* SQL: Move shared REST client methods to shared-client
This commit is a preliminary step for moving JDBC to the REST client. It extracts the common REST clients from CLI and moves it to shared-client. This will allow us to move to the 5 project setup: rest-proto, shared-client, server, jdbc, cli with the following dependencies:
server <-- rest-proto
shared-client <-- rest-proto
jdbc <-- shared-client
cli <-- shared-client
Relates to elastic/x-pack-elasticsearch#3419
Original commit: elastic/x-pack-elasticsearch@2e6a134de0