This commit adds compatibility testing of our JDBC driver against
different Elasticsearch versions. Although we are really testing the
forwards compatibility nature of the JDBC driver we model the testing
the same as we do existing BWC tests, that is, with the current branch
fetching the earlier versions of the artifact that is to be tested. In
this case, that's the JDBC driver itself.
Because the tests include the JDBC driver jar on it's classpath we had
to change the packaging of the driver jar in order to avoid jarhell and
other conflicting dependency issues when using an old JDBC driver with
later branches. For this we simply relocate all driver dependencies in
the shadow jar under a "shadowed" package. This allows the JDBC driver
to use the correct version of Elasticsearch libs classes, while the
tests themselves use their versions. Since this required a change to the
driver jar compatibility testing can only go back as far as that version
which at the time of this commit is 7.8.1.
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