AddMetadata should be able to clear existing fields

[vigoo]

Feature request

I would like to modify wasm_metadata's AddMetadata struct to be able to express clearing some of the existing metadata fields.

Why

The example use case I'm having is that previously I was storing custom name and version in WASM components as part of our component build flow, and by a recent change in cargo-component, it is now storing the crate name and version (from Cargo.toml) in the same metadata fields. I want our build flow to still store our name and version but unfortunately I am also rely on not having version in some cases (actually I'm storing the wit root package name and version - see WebAssembly/component-model#488). So I need to be able to clear the version metadata put there by cargo-component, if there was no version part of the root wit.

Proposal

I made this change in a fork by introducing something like

pub enum AddMetadataField<T: Debug + Clone> {
    Keep,
    Clear,
    Set(T),
}

and using this in place of the AddMetadata struct's Option fields, happy to open a PR if needed.

[alexcrichton]

This seems reasonable to me to add, thanks! And yes I'm happy to review a PR too. Bonus points if this can be integrated into the CLI too, but that's not required.