Simplify antlr4 runtime integration

[jeremyong]

• Adds o3de/antlr4 as a generation time dependency
• Removes unneeded python script to fetch git repos
• Adds antlr4_static as a target dependency
• Streamlines cmake options needed to ensure MSVC link compatibility

[jeremyong]

Note, after pulling this change down, src/external/antlr4 should be removed

[jeremyong]

The tests pass locally on Windows, so I'll have to investigate why the linux CI is complaining (edit: fixed)