[Enh]: Add support for `concat(..., how={"vertical_relaxed", "diagonal_relaxed"})`

[FBruzzesi]

Opening an issue to track the reason why we are not moving forward with #3191

To implement the relaxed variants, we need to implement the casting rules between dtypes (See #3191 (comment))

[dangotbanned]

You probably know already that I'm quite keen to support polars's supertyping rules 😄

Just thought I'd add that it is on my list for #2572:

• Review DataType propagation and supertyping rules
  • It's looking like some version of this will be required to correctly rewrite LogicalPlans
  • Otherwise, each sub-plan has to be eager and request a new schema from the backend after processing the previous step
    • That's current narwhals behavior, but likely more of an issue when Selectors can be used in more places
• A little write-up comparing oh-nodes to polars internals

I'm more than happy to help out/work on it if anyone else is interested