feat: make error clonable

[nathanosdev]

Overview This makes candid::Error clonable, which makes it easier for downstream clients to also make their Error types cloneable.

I've made a related PR for agent-rs.

Considered Solutions I can also work around by using Arc<T> in downstream clients.

Considerations This shouldn't have any impact on security or performance. Anyone that was interacting directly with the Custom variant may have a breaking change since it is wrapped in an Arc<T> now.