api-macro: support external types

api-macro: support external types

See the test example:

assuming a `pub struct Foo` which implements `Serialize` and
`Deserialize`, we also expect it to provide a
`pub const Foo::API_SCHEMA: &Schema` like so:

    #[derive(Deserialize, Serialize)]
    pub struct StrongString(String);
    impl StrongString {
        pub const API_SCHEMA: &'static Schema =
            &StringSchema::new("Some generic string")
                .format(&ApiStringFormat::Enum(&["a", "b"]))
                .schema();
    }

Then we can use:

    #[api(
        input: {
            properties: {
                arg: { type: StrongString },
            }
        },
        ...
    )]
    fn my_api_func(arg: StrongString) -> Result<...> {
        ...
    }

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>