Auto merge of #14683 - dacianpascu06:355adcad7, r=epage

Added unstable-schema generation for Cargo.toml

### What does this PR try to resolve?

Added unstable-schema feature that generates JsonSchema for Cargo.toml

See #12883
finished first step of [plan](#12883 (comment))

### Information
In cargo-util-schemas,  run cargo test --feature unstable-schema  . If there have been any changes to manifest.schema.json, rerun with env variable SNAPSHOTS=overwrite  and it will update them.

### How should we test and review this PR?
In cargo-util-schemas run cargo test --features unstable-schema and it will generate manifest.schema.json

Author: bors