ORDER BY pushdown into connectors

[lhofhansl]

We should support pushing down ORDER BY into connectors (such as JDBC, etc), and only do a streaming merge sort in Trino.

The use case I have in mind is reading data from (say) a Phoenix table along an index, the client would like to get streaming results, i.e. start reading from the resultset without the entire sort being materialized in Trino.
If Trino can push ORDER BY to a connector and that connector can decide (or not) to use an index, then that should be possible.

Another option is to (somehow) expose connector level indexes to Trino's planner.

Related to #18