MINOR: Add accessor methods to DateTimeIntervalExpr

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Ballista needs this so that it can serialize this expression

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #3440 (7ce6966) into master (73447b5) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3440      +/-   ##
==========================================
+ Coverage   85.49%   85.61%   +0.12%     
==========================================
  Files         296      297       +1     
  Lines       54331    54500     +169     
==========================================
+ Hits        46448    46658     +210     
+ Misses       7883     7842      -41     

Impacted Files	Coverage Δ
...tafusion/physical-expr/src/expressions/datetime.rs	84.65% <100.00%> (+0.43%)	⬆️
datafusion/physical-expr/src/planner.rs	92.68% <0.00%> (-0.87%)	⬇️
datafusion/sql/src/planner.rs	80.92% <0.00%> (-0.76%)	⬇️
datafusion/optimizer/src/reduce_outer_join.rs	98.19% <0.00%> (-0.61%)	⬇️
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	77.02% <0.00%> (-0.17%)	⬇️
datafusion/physical-expr/src/aggregate/count.rs	98.05% <0.00%> (ø)
datafusion/physical-expr/src/aggregate/average.rs	94.96% <0.00%> (ø)
...tafusion/physical-expr/src/aggregate/covariance.rs	98.70% <0.00%> (ø)
...sion/physical-expr/src/aggregate/count_distinct.rs	95.00% <0.00%> (ø)
... and 14 more

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[ursabot]

Benchmark runs are scheduled for baseline = 5af707b and contender = 1ec67da. 1ec67da 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