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.

[chenyan-dfinity]

This is a breaking change. It's quite labor intensive to bump the candid major version in the mono repo. So we try to avoid breaking changes as much as possible.

I can see two options: 1) we can let this PR to sleep for a while, until we make a major release (which can take quite a long time); 2) Candid error is not really structured. For the downstream users, it's enough to convert to err.to_string(), and clone as needed.

[nathanosdev]

I agree that making a major bump just for this feature is not worth the effort, so I'm fine with leaving this to sleep and in the meantime I will just to_string() downstream.

[ilbertt]

In addition to the previous discussion, in Rust the errors are not typically clonable. Closing it.