feat: publish shaded jar on maven central

[olavloite]

This is a copy of googleapis/google-cloud-java#6957.

Includes the shaded Spanner JDBC jar in the standard build for the library and publishes it to Maven central with a classifier. This makes the shaded jar downloadable directly from Maven, instead of requiring users to build the jar themselves. It also makes it possible to add automated tests for whether the shaded jar works or not.

The profile build-jdbc-driver is removed and the execution of the shade build goal that was part of the profile is moved into the default build profile. This ensures that the shaded jar is always built.

Fixes #22

[codecov]

Codecov Report

Merging #83 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #83   +/-   ##
=========================================
  Coverage     71.95%   71.95%           
  Complexity     1027     1027           
=========================================
  Files            24       24           
  Lines          3156     3156           
  Branches        463      463           
=========================================
  Hits           2271     2271           
  Misses          706      706           
  Partials        179      179           

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