java CDK: reduce dependency bloat

[postamar]

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My recent investigations in profiling the performance of a source-postgres spec call in #31298 revealed that having less jars on the classpath might speed up the JVM startup and class initialization latency slightly.

The principal source of bloat is the CDK which just includes dependencies for everything. This was a fine choice for the purpose of getting the CDK out of the door in the first place but now we should improve things.

It also makes no sense for the source-postgres base image to weigh in at half a GB, this bloat has repercussions elsewhere as well (the build is slower, it just looks bad, etc.)