fix: ResultSet#get(...) methods should auto convert values

[olavloite]

Methods like ResultSet#getString(int) and ResultSet#getInt(int) would only allow data to be fetched if the underlying data type matched the method that was invoked, i.e. calling ResultSet#getString(int) for a column containing an INT64 value would always fail. According to the JDBC specification it should however be allowed to call certain get... methods that automatically convert the underlying value to the requested type. The supported conversions will in some cases always be supported (e.g. INT64 to STRING), and in some cases the method may throw an exception if the actual value cannot be converted to the requested type (e.g. STRING to INT64).

This PR adds support for all implicit conversions for ResultSet#get... methods that according to the JDBC specification should be allowed. This also enables the use of the JDBC driver with frameworks that rely on these automatic conversions, such as MyBatis.

[codecov]

Codecov Report

Merging #143 into master will increase coverage by 0.24%.
The diff coverage is 83.70%.

@@             Coverage Diff              @@
##             master     #143      +/-   ##
============================================
+ Coverage     71.95%   72.20%   +0.24%     
- Complexity     1027     1095      +68     
============================================
  Files            24       24              
  Lines          3156     3292     +136     
  Branches        463      495      +32     
============================================
+ Hits           2271     2377     +106     
- Misses          706      722      +16     
- Partials        179      193      +14     

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/jdbc/JdbcTypeConverter.java	73.79% <50.00%> (-1.07%)	91.00 <3.00> (+3.00)	⬇️
...a/com/google/cloud/spanner/jdbc/JdbcResultSet.java	78.81% <81.14%> (-3.04%)	186.00 <10.00> (+57.00)	⬇️
...google/cloud/spanner/jdbc/AbstractJdbcWrapper.java	77.31% <100.00%> (+13.31%)	51.00 <8.00> (+8.00)

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