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

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

- [ ] https://github.com/apache/iceberg-rust/issues/1819
- [x] https://github.com/apache/iceberg-rust/issues/1314

Start by reviewing linked issue #1819 and the completed issue #1314 to understand the existing work and boundaries. Define what separating planning from execution requires, including the user-provided file I/O and execution-engine abstractions. Done means an agreed kernel design that makes those components pluggable.

Written by the indexing model from the issue text.