feat: support unit and struct enums

[the-wondersmith]

PR adds support for unit structs and struct-style enums as subcommands to argh::FromArgs.

Note

PR created as draft until #239 lands, as the branch is based off the-wondersmith/argh:feat/skip-field-attribute.

Specifically, this allows argh consumers to directly define simple CLIs as struct-style enums, e.x. something like netavark's plugin contract:

/// An example plugin for netavark
#[derive(Clone, Debug, argh::FromArgs)]
#[argh(help_triggers("-h", "--help", "help"))]
pub enum MyNetavarkPlugin {
    /// report the plugin's version and API version as `netavark`-compatible JSON.
    Info,

    /// create a `podman` network
    Create {
        /// the network definition to be created
        ///
        /// > Note: netavark streams this as JSON on STDIN.
        /// > This positional arg is defined so that manual
        /// > invocations (i.e., testing) are more convenient.
        #[argh(positional, from_str_fn(from_arg_or_stdin))]
        network: serde_json::Value,
    },

    /// plumb a container's veth and reserve its address(es)
    Setup {
        /// the container's network namespace path.
        #[argh(positional)]
        namespace: String,

        /// configuration of the container+network to set up.
        ///
        /// > Note: netavark streams this as JSON on STDIN.
        /// > This positional arg is defined so that manual
        /// > invocations (i.e., testing) are more convenient.
        #[argh(positional, from_str_fn(from_arg_or_stdin))]
        network: serde_json::Value,
    },

    /// release a container's reserved address(es) and unplumb its veth
    Teardown {
        /// the container's network namespace path.
        #[argh(positional)]
        namespace: String,

        /// configuration of the container+network to tear down.
        ///
        /// > Note: netavark streams this as JSON on STDIN.
        /// > This positional arg is defined so that manual
        /// > invocations (i.e., testing) are more convenient.
        #[argh(positional, from_str_fn(from_arg_or_stdin))]
        network: serde_json::Value,
    },
}

Also, subcommands that take no arguments (e.g. #157) become simpler to define:

/// An argumentless subcommand
#[derive(Debug, PartialEq, argh::FromArgs)]
#[argh(subcommand)]
struct SomeSimpleCommand;  // currently, would be required to be `SomeSimpleCommand { }`

PR includes comprehensive tests for added types/variants, test suite is green across the board (checked with cargo hack --feature-powerset test --workspace --all-targets -- --include-ignored [ref: cargo hack]).