fix: add support for WITH clauses

[olavloite]

Cloud Spanner now also support WITH clauses in a SELECT statement. These clauses were not recognized by the JDBC driver.

Fixes #41

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b368b84). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #42   +/-   ##
=========================================
  Coverage          ?   77.83%           
  Complexity        ?     1721           
=========================================
  Files             ?       60           
  Lines             ?     5866           
  Branches          ?      825           
=========================================
  Hits              ?     4566           
  Misses            ?      926           
  Partials          ?      374

Impacted Files	Coverage Δ	Complexity Δ
...com/google/cloud/spanner/jdbc/StatementParser.java	87.97% <100%> (ø)	46 <0> (?)

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[hengfengli]

LGTM. Good to see this supported by spanner.