[epic] Bring in maven plugin, archetype and java codegen

[octonato]

As discussed, we plan to merge into this project the maven plugin, the maven archetypes and the java codegen.

The execution plan is as follow:

Phase 1:

• move: bring maven-java under java-sdk (preserving commit history) Merging the maven-java project into SDK #28
• build: must be integrated in CircleCI (similar to what we are doing to the samples already) Integrate maven-java into CI build #36
• versioning: we want the maven plugin and archetypes to have the same version of the java-sdk. build publish maven plugin from CI to sonatype #65
• publishing: archetypes and plugin must be published under a new name because of version misalignment fix: use com.akkaserverless groupId and renamed artifacts #45 and build publish maven plugin from CI to sonatype #65
• docs: documentation must be adapted since users won't need to change anything in their settings.xml file and we will have a new artifact name docs: remove Maven settings.xml instructions #96
• github issues: move all open issues from maven-java to java-sdk

Phase 2:

First we need to split codegen into codegen-core, codegen-java and codegen-js (preserving commit history) See https://github.com/lightbend/akkaserverless-codegen/issues/147

• move codegen-java under java-sdk (preserving commit history)

Please, feel free to add here any other point of action that you believe we will need.

[octonato]

Since the move may take a few iterations, I suggest that we create a feature branch.