feat: make error clonable

[nathanosdev]

Description

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

It's also possible to workaround by using Arc<T> in downstream clients.

I've made a related PR for Candid.

I'm currently referencing Candid by git in Cargo.toml, but of course I'll remove that if the corresponding change is agreed upon and merged in Candid.

Checklist:

• [ ] The title of this PR complies with Conventional Commits.
• [ ] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.