Moves Junit 4 support from core to separate module

[froque]

Module is named testcontainers-junit4 (as suggested in #737) instead of junit4 because it fails when building examples with "> Module version 'org.testcontainers:junit4' is not unique in composite: can be provided by [project :testcontainers-java:docs:examples:junit4, project "

Seems related to https://github.com/gradle/gradle/issues/12872

Fixes #970 and #87 (duplicates #2826, #3101, #3374, #3548, #3737, #3753, #3756, #5287, #5404, #1525)

This is a breaking change and should target future testcontainers-2.0.

Could not find any branch for future 2.0 so this pull request is targeting master.

I started working on a openrewrite recipe to acompany this pull request but is not yet presentable.