docs: add guide to adding a new expression

[tshauck]

Which issue does this PR close?

Part of #370 (haven't added anything on cast or aggregate, mainly scalar func)

Rationale for this change

Make it easier for people to contribute new expressions.

What changes are included in this PR?

A new markdown file in the contributor guide.

How are these changes tested?

~Not yet, I haven't tried building the docs. Will do before marking this PR ready.~

Built the docs and reviewed the HTML.

[tshauck]

Notes from comet community meeting:

• Possible to get link for presentation slides and video?
  • Andy to publish video
  • Liang Chi has slides here: https://docs.google.com/presentation/d/1H0fF2MOkkBK8fPBlnqK6LejUeLcVD917JhVWfp3mb8A/edit#slide=id.p
• CometSparkSessionExtension injects rules into spark session extension
  • E.g. replace spark scan with comet scan
  • Replace spark execution with comet execution class
• QueryPlanSerde takes spark expression an serializes via proto
• Cast/types support is not fully supported (e.g. Add(left, right, _) checks if supportedDataTypes(left.dataType).
• operator.proto defines Operator
• message Expr... need to update expr.proto (See Add for example)
  • left, right, return type
• PhysicalPlanner is the physical planner struct
  • create_expr and create_plan are two most important parts
  • gets children from protobuf message and calls create_expr or create_plan to generate the physical plan
  • spark_expr.expr_struct is matched on then passed to appropriate part
  • Can sometimes use built-in support for expressions (e.g. is not null expression), this only works if the implement is compatible w/ spark (e.g. cast)
• Adding a Cast
  • datafusion's PhysicalExpr is implemented for Cast.
• Spark version differences (e.g. failOnError is a common change from 3.3 to 3.4+)
  • use shim layer to handle API differences
• CometExpressionSuite is for testing
  • use checkSparkAnswerAndOperator

Questions:

• Which expressions to support? Try to get support for the full expression.
  • 3.4 main difference is failOnError
  • Expect 4.0 to be reasonably difference
• How to handle new expressions? Add new expression into datafusion if very common expression with standard behavior
• Which other areas are important? Aiming for complete coverage... CAST is recently a hotbed of activity and high priority... tests are important
• Unrelated to expressions: first release? Andy has started copying over the basic datafusion scripts to get out a source release, but ultimate goal is jar file with compiled code.

[andygrove]

@tshauck video link is https://drive.google.com/file/d/1POU4lFAZfYwZR8zV1X2eoLiAmc1GDtSP/view?usp=sharing

[tshauck]

I think this is ready for review. The first draft I wrote was pretty slanted towards scalar function expressions since that's what I did w/ unhex, but after the meeting on Wed, I made some updates so hopefully it's more general. I'm not 100% sure on the things I haven't touched yet, so certainly open to feedback on any of it.

Here's what it looks like included in the docs...

[andygrove]

This is looking great @tshauck. Could you fix the merge conflict?

[tshauck]

Thanks, @andygrove -- I think the conflict is resolved now.

[tshauck]

@andygrove Conflict should actually be fixed now, thanks @kazuyukitanimura