DRILL-7711: Add data path, parameter filter pushdown to HTTP plugin
Adds an option to specify the path to data so the plugin will
ignore REST message "overhead" except the actual data.
Allows specifying HTTP URL parameters as filter push-downs from
SQL.
* Revised scan cost model to allow a scan more freedom
to define cost. Needed to ensure that filter push-down
is actually accepted by Calcite.
* Reduced size of HTTP scan plan by including on the one
needed connection config rather than all of them.
* Revised URL building so that the proxy sees the full
URL after adding parameters, etc.
* Many code refinements.
* Added more details to README
