refactor(java): Streamline ClassResolver responsibilities

[LiangliangSui]

To reduce the complexity of the ClassResolver class, we mainly do the following two things:

1. Move ClassId related responsibilities into ClassIdAllocator class.

2. Move registeredId2ClassInfo into ExtRegistry static inner class since they are content related.

[chaokunyang]

Hi @LiangliangSui , thanks for this work. We're standardlizing Fury xlang serialization protocol in #1413 , which may change class resolver too, especially the class id part. Can we hold this until the xlang serialization protocol are merged, so we don't have to refactor ClassResolver again.

The type id is lift to the first-class concept in #1413, we do need to strealine it from class resolver.

[LiangliangSui]

Ok, we keep this PR until the xlang serialization protocol is merged.