idea: Build Iceberg Kernel to make it more extensible for users

[Xuanwo]

I think it’s a great idea to split iceberg-kernel so we can separate the planning and execution stages.

We can follow delta-rs’s pattern to build it, providing a good abstraction for users to implement their own engine. In this kernel, we’ll let users plug in their own file IO and execution engine.

Tasks

• Split iceberg-kernel #1819
• Make FileIO a Trait #1314