Add ExampleReadWalkerWithVariantsSpark.java and tests

[tomwhite]

Includes a test for the non-Spark version, and tests the two produce the same result.

Fixes #5276.

[droazen]

Reassigning to @jamesemery

[jamesemery]

A pretty simple set of tests. I don't see a particular reason not to approve this

[codecov-io]

Codecov Report

Merging #5289 into master will increase coverage by 0.318%.
The diff coverage is 83.636%.

@@               Coverage Diff               @@
##              master     #5289       +/-   ##
===============================================
+ Coverage     86.776%   87.094%   +0.318%     
- Complexity     29888     30529      +641     
===============================================
  Files           1834      1859       +25     
  Lines         138473    141539     +3066     
  Branches       15241     15540      +299     
===============================================
+ Hits          120162    123272     +3110     
+ Misses         12760     12620      -140     
- Partials        5551      5647       +96

Impacted Files	Coverage Δ	Complexity Δ
...itute/hellbender/engine/spark/ReadWalkerSpark.java	88.889% <ø> (+16.667%)	11 <0> (+3)	⬆️
.../ExampleReadWalkerWithVariantsIntegrationTest.java	100% <100%> (ø)	3 <3> (?)
...pleReadWalkerWithVariantsSparkIntegrationTest.java	100% <100%> (ø)	3 <3> (?)
...s/examples/ExampleReadWalkerWithVariantsSpark.java	68.966% <68.966%> (ø)	6 <6> (?)
...ls/funcotator/metadata/VcfFuncotationMetadata.java	71.429% <0%> (-28.571%)	8% <0%> (+3%)
...hellbender/tools/walkers/vqsr/CNNVariantTrain.java	60% <0%> (-20.645%)	4% <0%> (ø)
...der/tools/walkers/annotator/RMSMappingQuality.java	83.607% <0%> (-10.208%)	41% <0%> (+1%)
...der/tools/walkers/mutect/M2ArgumentCollection.java	87.5% <0%> (-6.944%)	8% <0%> (+5%)
...itute/hellbender/tools/walkers/mutect/Mutect2.java	81.395% <0%> (-6.483%)	23% <0%> (+4%)
...bender/tools/walkers/mutect/FilterMutectCalls.java	92.5% <0%> (-4.722%)	14% <0%> (+2%)
... and 149 more