ItemId: API design

[azriel91]

Enables developers to not use strings, and peace to not clone everywhere.

Currently ItemId is a Cow<'_, str> newtype. If we instead require developers to provide something like:

#[derive(ItemIds)]
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum EnvDeployFlowItemIds {
    ProfileS3Bucket,
    WebAppDownload,
    WebAppS3ObjectUpload,
}

for which peace generates:

// for each variant
#[derive(Deserialize, Serialize)]
pub struct ProfileS3Bucket;

impl EnvDeployFlowItemIds From<ProfileS3Bucket> {}
impl ProfileS3Bucket TryFrom<EnvDeployFlowItemIds> {}
impl Display for ProfileS3Bucket {} // maybe

then code will be a bit more robust, though it does entail defining the enum everywhere you write tests.

Need to also consider upgrades: additions, removals, and renames.