Implement IS UNKNOWN/IS NOT UNKNOWN operators

[isidentical]

Which issue does this PR close?

Closes#3195.

Rationale for this change

This PR implements IS UNKNOWN and IS NOT UNKNOWN SQL operations (similar to #3235).

What changes are included in this PR?

The transformation of SQL-level IS UNKNOWN / IS NOT UNKNOWN operations to IS DISTINCT FROM queries in the logical level.

Are there any user-facing changes?

2 new operators.

@andygroveI assume that just like #3225, this would also require some type-checking. Should we implement it afterwards, or should we wait your PR and I can make the required changes in this PR.

[isidentical]

Also CC: @sarahyurick

[andygrove]

Thanks @isidentical! I think we can go and merge this and then add the type checking once my PR is merged (which could take a while). I filed #3247 to track this.

[isidentical]

Thanks @andygrove! I've subscribed to your PR, so hopefully once it is merged I can start working on the type-checking phase in another PR.

[codecov-commenter]

Codecov Report

Merging #3246 (5aa0d2a) into master (bdc56ca) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 5aa0d2a differs from pull request most recent head 225aebe. Consider uploading reports for the commit 225aebe to get more accurate results

@@ Coverage Diff @@## master #3246 +/- ##
=======================================
Coverage 85.84% 85.85% =======================================
Files 293 293 Lines 53246 53284 +38 =======================================
+ Hits 45710 45747 +37 - Misses 7536 7537 +1 

Impacted Files	Coverage Δ
datafusion/core/tests/sql/expr.rs	99.85% <100.00%> (+<0.01%)	⬆️
datafusion/sql/src/planner.rs	80.59% <100.00%> (+0.07%)	⬆️
datafusion/expr/src/logical_plan/plan.rs	78.38% <0.00%> (-0.35%)	⬇️
datafusion/expr/src/window_frame.rs	93.27% <0.00%> (+0.84%)	⬆️

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[ursabot]

Benchmark runs are scheduled for baseline = bdc56ca and contender = 94dacce. 94dacce 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