Recent API change in `GlobalLimitExec` breaks compatibility with Ballista

[andygrove]

Describe the bug
Commit a255291 removed the public API method GlobalLimitExec::limit() which Ballista relies on in order to be able to introspect the operator and determine the limit.

To Reproduce
Upgrade Ballista to use DataFusion 9.0.0-rc1 and try compiling.

Expected behavior
We should maintain public methods for accessing limit (and offset as well now).

Additional context
None