Make `cargo.features` a crate-level config

workspace
|-crate1
|-crate2
|-crate3
|_crate4

crate 2 and 4 have a feature called "example_feature" that i want to enable for RA,
so using a workspace setting is a no go, because RA will complain that crates 1 and 3 don't have this feature
creating rust-analyzer.toml in crate 2 and 4's root and adding cargo.features = ["example_feature"] has no effect,
however if i replace it with rust-analyzer.cargo.features = ["example_feature"], RA will complain that this is an invalid key that doesn't exist. So it reads the file but when it reads a correct key it doesn't apply that? I have no machine-wide or workspace or user overrides (using VSC right now but same happens in Vim).

Originally posted by @vaqxai in https://github.com/rust-lang/rust-analyzer/issues/13529#issuecomment-2345668435

What the title indicates is just a way to resolve this issue although making such a change is not wanted. This issue is just to discuss and shed a light on what we can do

Start by reproducing the behavior described for crate-level rust-analyzer.toml files in crates 2 and 4, comparing cargo.features with rust-analyzer.cargo.features in VS Code or Vim. Done means determining and documenting a supported way to apply a feature only to selected workspace crates without invalid-key errors or affecting crates that lack the feature.