Convert to multi-module project (separate models and api)

[jmini]

With version 6 we will split the project into a multi module project:

• gitlab4j-models
• gitlab4j-api
• gitlab4j-api-jersey2 (that corresponds to the gitlab4j-api from version 5.x?). This module would be a copy from gitlab4j-api but with usage of the javax package.

This will allow usage with other libs (see #1011 where some user would like to use Ok-http)

This is also a good step to be able to be independent from the http lib (see #924)

[jmini]

Implemented with 6.0.0-rc7 without gitlab4j-api-jersey2 (I am not sure if somebody really needs it in 2024).

I hope users will try this RC version to gather feedback.