Allow fields to be omitted in registry index metadata

[kornelski]

This allows omission of some fields in the index metadata where there's an obvious default matching default of Cargo.toml.

• In the future, for crates with high enough MSRV, crates.io could omit these fields to save a few bytes in the index.
• It makes it a bit easier to implement custom registries, e.g. having to specify "optional": false for every dependency seems unnecessary, and having to emit features: {} when using features2 is surprising.

[rustbot]

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[epage]

This requires an issue, maybe even an RFC. Closing until this has been approved.