Function register_backend 

pub fn register_backend(kind: &str, build: BackendBuildFn) -> Result<()>

Register a constructor for a cache backend under kind.

Returns Err if a non-built-in kind is already registered. Built-in backends may be replaced so callers can mask a built-in implementation (for example, a patched "moka" backend) without changing URI/config strings elsewhere.

Typical usage from a backend crate:

fn build_my_backend(_config: &BackendConfig) -> Result<Arc<dyn CacheBackend>> {
    Ok(Arc::new(MokaCacheBackend::with_capacity(1024)))
}

register_backend("my-backend", build_my_backend)?;