Describe each format in Rust and remove plugin.toml #444

Part of #442.

Where things stand (after #437)

Each format crate has a plugin.toml next to its Cargo.toml:

[plugin]
id = "nuget"
name = "NuGet"
version = "1.0.0"
api = "0.3.0"
description = "NuGet V3 feeds: …"
mount = "/v3"
kinds = ["hosted", "proxy", "group"]

[assets]
package = "package.nupkg"
package_type = "application/octet-stream"
manifest = "package.nuspec"
manifest_type = "application/xml"

The crate hands this file to the server as text: pub const PLUGIN_TOML: &str = forge_plugin_api::plugin_toml!();, which is an include_str!. The server parses it at startup:

• Plugin::builtin(PLUGIN_TOML, Nuget::with_limits(…)).expect("plugins/nuget/plugin.toml is valid");
• Manifest::parse with the toml crate;
• Box::leak on the id to get a &'static str.

The file was the manifest an operator installed next to a WebAssembly component. Now that only built-in formats remain, it holds values the compiler could check, written in a second language:

• a typo in a key or a kind fails at startup instead of at build time;
• version repeats the crate's version, and a unit test in each crate keeps the two equal;
• api can only be forge_plugin_api::API, because the contract and the formats build together and the compiler already checks the trait;
• every format crate repeats the same plugin_toml_names_this_crate test.

Proposal

Each format describes itself in Rust, as a 'static value that the trait returns:

pub const DESCRIPTOR: Descriptor = Descriptor {
    id: "nuget",
    name: "NuGet",
    version: env!("CARGO_PKG_VERSION"),
    description: "NuGet V3 feeds: …",
    kinds: &[RepositoryKind::Hosted, RepositoryKind::Proxy, RepositoryKind::Group],
    assets: Assets {
        package: "package.nupkg",
        package_type: "application/octet-stream",
        manifest: "package.nuspec",
        manifest_type: "application/xml",
    },
};

impl FormatPlugin for Nuget {
    fn descriptor(&self) -> &'static Descriptor {
        &DESCRIPTOR
    }
    // …
}

• Descriptor (the name is open) goes in forge-plugin-api, next to RepositoryKind and Assets. The fields of Assets become &'static str, and an Assets::DEFAULT replaces the serde default. The descriptor comes from a trait method rather than an associated const because the server holds formats as Arc<dyn FormatPlugin>.
• Plugin::builtin(implementation) takes only the format. Plugin::manifest() becomes descriptor(), and Plugin::id() returns descriptor().id with no leak.
• The startup parser validated the id's spelling, the asset names and the reserved ids (RESERVED_IDS: docker and the server's routes). Those checks move into a test over plugins::builtin() in forge-server, together with a check that no two formats share an id. A bad descriptor then fails cargo test instead of startup.
• storage from Raw repositories as a format plugin: a file store in the plugin contract #431 becomes a descriptor field like the others, and so does mount until Serve every repository at /repository/{name}/, with no format in the URL (breaking) #438 removes it. If Serve every repository at /repository/{name}/, with no format in the URL (breaking) #438 lands after this, it removes the mount field instead of the manifest key, and its criterion that a plugin.toml with mount is refused no longer applies.
• Remove:
  • plugin.toml from every format crate;
  • plugin_toml! and PLUGIN_TOML;
  • Manifest::parse and Manifest::load, PluginError::Manifest and SUPPORTED_APIS;
  • forge_plugin_api::API and the api key;
  • the plugin_toml_names_this_crate tests;
  • the "keep it equal to version in plugin.toml" comments in the crates' Cargo.toml;
  • forge-plugin's toml dependency.
• Update the docs: the crates' README and Writing a plugin in docs/format-plugins.md show the descriptor instead of plugin.toml.

Alternatives considered

• Keep plugin.toml. Only the crate it sits in reads it. Keeping it keeps the startup parse and the duplicated version.
• Use [package.metadata.forge] in Cargo.toml. This saves one file, but the values are still TOML that something parses at runtime or in a build script, with no type checking.

No changelog fragment is needed, since nothing changes for anyone running Forge. GET /admin/formats answers as before.

Acceptance criteria

• There is no plugin.toml in the repository, and git grep -n 'plugin\.toml\|plugin_toml\|PLUGIN_TOML' finds nothing outside CHANGELOG.md and docs/changes/.
• cargo tree -p forge-plugin shows no toml.
• GET /admin/formats returns the same ids, names, versions, descriptions and kinds as before, and the existing admin tests pass unchanged.
• A test over the built-in formats refuses a malformed or reserved id, two formats with the same id, and invalid asset names.
• The docs show a format's descriptor, not plugin.toml.

Blocked by #437: a WebAssembly component needs its manifest on disk, so the file has to stay for as long as components can load. This can land before or after #443, but not at the same time, because both change the same files.