Refactor Expr::Case to use a struct

[andygrove]

Which issue does this PR close?

Part of #2175

Rationale for this change

There were recent discussions (see #2175) about making Expr more consistent with LogicalPlan where variants wrap structs. There seemed to be consensus that this was the direction we want to go.

I have not seen anyone start on this so thought I would get the ball rolling.

This PR applies this for Expr::Case.

What changes are included in this PR?

Are there any user-facing changes?

[andygrove]

@mingmwang @yjshen @jackwener fyi

[alamb]

LGTM

[alamb]

If you wanted to avoid &case I think you could do like

Suggested change

	Expr::Case(case) => {
	Expr::Case(Case {
	expr,
	when_then_expr,
	else_expr,
	}) => {

Maybe the & is nicer syntax 🤔

[andygrove]

I debated between the two styles too. I don't have a strong preference either way

[ursabot]

Benchmark runs are scheduled for baseline = 567ca5d and contender = 488b2ce. 488b2ce is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java