refactor: use Connection API in client lib

[olavloite]

The generic Connection API has been moved to the client library. The JDBC driver still contained its own copy of this API. This change removes this own API and instead uses the one in the client library.

This also changes the dependency management of the JDBC driver from self-management into using the com.gooble.cloud.libraries-bom.

[codecov]

Codecov Report

Merging #133 into master will decrease coverage by 5.96%.
The diff coverage is 6.89%.

@@             Coverage Diff              @@
##             master     #133      +/-   ##
============================================
- Coverage     77.92%   71.95%   -5.97%     
+ Complexity     1732     1027     -705     
============================================
  Files            60       24      -36     
  Lines          5911     3156    -2755     
  Branches        835      463     -372     
============================================
- Hits           4606     2271    -2335     
+ Misses          928      706     -222     
+ Partials        377      179     -198     

Impacted Files	Coverage Δ	Complexity Δ
...gle/cloud/spanner/jdbc/AbstractJdbcConnection.java	65.27% <ø> (ø)	21.00 <0.00> (ø)
...ogle/cloud/spanner/jdbc/AbstractJdbcStatement.java	54.31% <ø> (ø)	29.00 <0.00> (ø)
.../com/google/cloud/spanner/jdbc/JdbcDataSource.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...oogle/cloud/spanner/jdbc/JdbcDatabaseMetaData.java	95.23% <ø> (ø)	180.00 <0.00> (ø)
...java/com/google/cloud/spanner/jdbc/JdbcDriver.java	61.19% <ø> (+34.32%)	14.00 <0.00> (+7.00)
...ogle/cloud/spanner/jdbc/JdbcPreparedStatement.java	69.23% <ø> (ø)	7.00 <0.00> (ø)
...a/com/google/cloud/spanner/jdbc/JdbcStatement.java	67.74% <ø> (ø)	38.00 <0.00> (ø)
...ava/com/google/cloud/spanner/jdbc/SpannerPool.java	0.00% <0.00%> (-86.93%)	0.00 <0.00> (-30.00)
...e/cloud/spanner/jdbc/TransactionRetryListener.java	0.00% <0.00%> (-100.00%)	0.00 <0.00> (ø)
.../com/google/cloud/spanner/jdbc/JdbcConnection.java	49.72% <7.40%> (-8.50%)	42.00 <2.00> (ø)
... and 2 more

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