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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