feat: add struct query parameters

[pmakani]

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Fixes #203

[codecov]

Codecov Report

Merging #223 into master will increase coverage by 0.23%.
The diff coverage is 95.83%.

@@             Coverage Diff              @@
##             master     #223      +/-   ##
============================================
+ Coverage     77.36%   77.59%   +0.23%     
- Complexity     1116     1146      +30     
============================================
  Files            73       75       +2     
  Lines          5950     6074     +124     
  Branches        626      648      +22     
============================================
+ Hits           4603     4713     +110     
+ Misses         1019     1017       -2     
- Partials        328      344      +16

Impacted Files	Coverage Δ	Complexity Δ
...com/google/cloud/bigquery/QueryParameterValue.java	81.77% <95.83%> (+9.04%)	60 <3> (+14)	⬆️
...om/google/cloud/bigquery/LoadJobConfiguration.java	87.02% <0%> (-2.62%)	52% <0%> (ø)
...google/cloud/bigquery/HivePartitioningOptions.java	93.75% <0%> (ø)	9% <0%> (?)
...gle/cloud/bigquery/MaterializedViewDefinition.java	70% <0%> (ø)	6% <0%> (?)
...ava/com/google/cloud/bigquery/TableDefinition.java	87.09% <0%> (+0.88%)	8% <0%> (+1%)	⬆️

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