SQL Client gateway mode should accept URLs

Currently, the --endpoint parameter has to be specified in the InetSocketAddress  format, i.e. hostname:port. While this works fine for basic use cases, it does not support the placement of the gateway behind a proxy or using an Ingress for routing to a specific Flink cluster based on the URL path.  I.e. it expects some.hostname.com:9001  to directly serve requests on some.hostname.com:9001/v1 . Mapping to a non-root location, i.e. some.hostname.com:9001/flink-clusters/sql-preview-cluster-1/v1  is not supported.
 
Since the client talks to the gateway via its REST endpoint, the right format for the --endpoint  parameter is URL, not InetSocketAddress .  

The same --endpoint parameter can be reused if the changes are implemented in a backwards-compatible way.