Expose per-crate cfg sets in scarb metadata

[mkaput]

Problem

Cairo 2.5.0 introduced per-crate cfg sets (starkware-libs/cairo#4831). We need to include them in scarb-metadata output.

Proposed Solution

Add a new field to CompilationUnitComponentMetadata:

#[serde(default)]
pub cfg: Vec<Cfg>

and populate it with cfg items to-be-attached to this unit.

The global per-unit cfg items set should get main component's cfg items, for backwards compatibility reason.

Notes

1. Don't forget about extensions that are pulling information from.
2. Make scarb-metadata release after merging these changes.