External Data Source: Public API

Adds the thrift structures for the public external data source API
and a new maven project containing the Java ExternalDataSource
interface and the generated Java thrift classes.

The ExternalDataSource.thrift structures can evolve in a backward
compatible way. The ExternalDataSource Java interface will always
contain a version number in the namespace (e.g.
com.cloudera.impala.extdatasource.v1 for V1) so we can potentially
make breaking changes to the interface in the future but still
support older versions.

A trivial implementation of the ExternalDataSource API is also
added for testing purposes.
TODO: Make the sample data source implementation realistic.

Change-Id: I827d6420a87ed7a2bce34e050362ca98ddc5dbcc
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2241
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins