Improve Rust ergonomics in the codebase

[thevar1able]

Rust contribs are great addition to ClickHouse, and are not going anywhere anytime soon.
However, they are integrated rather poorly.

Things we'd like to improve:

• agree on using either rust/workspace approach, or contrib/ approach
• crates which are shared between contribs should only compile once

[Algunenano]

agree on using either rust/workspace approach, or contrib/ approach

I think the workspace approach is much cleaner. The reason we couldn't add delta-kernel to it was that it has internal dependencies (via paths) and we couldn't make it work, but ideally we would put it there as it simplifies things quite a bit (specially dependency management).

crates which are shared between contribs should only compile once

I think having a workspace eliminates this, right? At least for common crate-version

[thevar1able]

we couldn't add delta-kernel

chdig too.

[thevar1able]

I don't understand what is so special about Rust that it deserved a separate root folder, but also I am not sure if it is possible to converge to contrib/ layout completely without losing crates cache, like it works in workspace.

That's why I filed this item 🙂