feat: add support connection properties for job and query

[pmakani]

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Fixes #306

[codecov]

Codecov Report

Merging #320 into master will increase coverage by 0.13%.
The diff coverage is 94.11%.

@@             Coverage Diff              @@
##             master     #320      +/-   ##
============================================
+ Coverage     77.56%   77.70%   +0.13%     
- Complexity     1158     1172      +14     
============================================
  Files            76       77       +1     
  Lines          6103     6153      +50     
  Branches        656      660       +4     
============================================
+ Hits           4734     4781      +47     
  Misses         1020     1020              
- Partials        349      352       +3     

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/bigquery/ConnectionProperty.java	91.89% <91.89%> (ø)	12.00 <12.00> (?)
...m/google/cloud/bigquery/QueryJobConfiguration.java	77.68% <100.00%> (+0.82%)	58.00 <1.00> (+2.00)

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