Prepare maven SNAPSHOT version

[jmini]

This PR prepares the possibility to deploy the artifacts to a maven repository (in addition to the docker image).

• Version is now 3.0.0-SNAPSHOT to add the possibility to perform releases *.Produced jar (in the target/ folder) is now openapi-petstore.jar instead of openapi-petstore-${version}.jar
• Add necessary maven metadata for publication on maven-central
• groupId changed to org.openapitools.petstore to be more consistent with other projects.
• Readme and Dockerfile are updated to reflect the changes