Publish artifacts back to Maven central

[Chrisss93]

#34 made jars and other artifacts be published exclusively to ConfluentHub. It would be a great help, and more standardized, if the build-process could reintroduce publishing artifacts back to maven central. Currently the README in this repo still points people to the maven published artifacts which are basically unchanged since the time this repo was forked from wepay.

[C0urante]

I believe the WePay engineers are interested in adding this back as well. @criccomini do you guys plan to add Maven Central publishing to the project in the near future?

[Chrisss93]

Hi, gentle reminder here. @criccomini is publishing artifacts back to maven central something WePay is looking to do? Thanks.

[C0urante]

Until/unless we address this, we should probably remove mention of Maven Central from the README. I've noted this in #89.

[chrisVia]

@C0urante Is this something that requires the original authors to move forward? I understand that Confluent has its own public maven repo for a select number of projects like kafka jdbcConnector. Would it be addressable by Confluent maintainers to add the artifacts there?

[C0urante]

Hi @chrisVia--unfortunately, it's not a priority for Confluent at the moment as we distribute our connectors via Confluent Hub. If that changes I'll be sure to let you know, though.

[C0urante]

I've taken a stab at this with #155 and #156. Because I don't work for WePay and don't want to publish artifacts to the com.wepay.kcbq namespace, we'll need someone from WePay to actually perform these releases, but hopefully after some GPG key and Sonatype credential wrangling, it should be as simple as running a single Maven command and then closing+promoting a staging repository in Sonatype's Nexus repo.

We might consider altering the project's groupId to io.confluent and integrating Maven Central releases into Confluent's CD pipeline for version 3.0.0 onward.

cc @criccomini and @mtagle--since you'd be responsible for performing releases on the 1.6.x and 2.x.y branches, can you take a look at the PRs and confirm that the release steps outlined in them work for you?