[BigQueryWorkloadTester] Adding BigQuery backend and test

[codecov[bot]]

Codecov Report

Merging #194 into dev will increase coverage by 34.31%. The diff coverage is 83.16%.

@@             Coverage Diff              @@
##               dev     #194       +/-   ##
============================================
+ Coverage     3.47%   37.78%   +34.31%     
- Complexity       7       42       +35     
============================================
  Files           15       19        +4     
  Lines          288      389      +101     
  Branches        10       12        +2     
============================================
+ Hits            10      147      +137     
+ Misses         277      240       -37     
- Partials         1        2        +1

Impacted Files	Coverage Δ	Complexity Δ
...tem/benchmark/backends/BigQueryBackendFactory.java	0% <0%> (ø)	0 <0> (?)
...ud/pontem/benchmark/backends/BackendException.java	100% <100%> (ø)	2 <2> (?)
...pontem/benchmark/backends/BigQueryBackendTest.java	90.47% <90.47%> (ø)	8 <8> (?)
...oud/pontem/benchmark/backends/BigQueryBackend.java	95.45% <95.45%> (ø)	10 <10> (?)
...gle/cloud/pontem/testing/BigQueryResultHelper.java	63.63% <0%> (+63.63%)	6% <0%> (+6%)	:arrow_up:
...va/com/google/cloud/pontem/testing/MapBuilder.java	66.66% <0%> (+66.66%)	3% <0%> (+3%)	:arrow_up:
... and 3 more