feat: add extract model for extractjobconfiguration

[pmakani]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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Towards #202

[codecov]

Codecov Report

Merging #227 into master will increase coverage by 1.10%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##             master     #227      +/-   ##
============================================
+ Coverage     77.36%   78.46%   +1.10%     
- Complexity     1116     1287     +171     
============================================
  Files            73       77       +4     
  Lines          5950     6728     +778     
  Branches        626      804     +178     
============================================
+ Hits           4603     5279     +676     
- Misses         1019     1080      +61     
- Partials        328      369      +41     

Impacted Files	Coverage Δ	Complexity Δ
...google/cloud/bigquery/ExtractJobConfiguration.java	93.18% <88.88%> (-1.32%)	39.00 <12.00> (+12.00)	⬇️
...va/com/google/cloud/bigquery/TimePartitioning.java	100.00% <0.00%> (ø)	12.00% <0.00%> (+5.00%)
...gle/cloud/bigquery/MaterializedViewDefinition.java	70.00% <0.00%> (ø)	6.00% <0.00%> (?%)
...ain/java/com/google/cloud/bigquery/PolicyTags.java	100.00% <0.00%> (ø)	5.00% <0.00%> (?%)
.../com/google/cloud/bigquery/ConnectionProperty.java	91.89% <0.00%> (ø)	12.00% <0.00%> (?%)
...google/cloud/bigquery/HivePartitioningOptions.java	83.78% <0.00%> (ø)	11.00% <0.00%> (?%)
...google/cloud/bigquery/StandardTableDefinition.java	93.90% <0.00%> (+0.23%)	16.00% <0.00%> (ø%)
...src/main/java/com/google/cloud/bigquery/Field.java	83.22% <0.00%> (+0.61%)	44.00% <0.00%> (+19.00%)
...m/google/cloud/bigquery/QueryJobConfiguration.java	78.84% <0.00%> (+1.98%)	75.00% <0.00%> (+19.00%)
...ain/java/com/google/cloud/bigquery/FieldValue.java	85.00% <0.00%> (+2.85%)	38.00% <0.00%> (+14.00%)
... and 5 more

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[gcf-merge-on-green]

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[shollyman]

Thanks!