Distributed RDataFrame: add support for NodeProxy.GetColumnNames

[gwmyers]

This Pull request:

Update the distributed RDataFrame to use the HeadNode _localrdf to keep track of transform operations, and add the ability to call GetColumnNames on a NodeProxy object.

Changes or fixes:

• Add GetColumnNames method to NodeProxy which calls the HeadNode GetColumnNames method
• Propagate transform operations to the HeadNode _localrdf, mainly to keep track of user-defined columns

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

This PR fixes #15442