UX: Cloneable traits [age crate]

[mberry]

For ergonomics it would nice to be able to clone the Identity and Recipient traits, had a few usecases where it becomes unwieldy without it.

Probably the easiest way to implement this is with https://github.com/dtolnay/dyn-clone,

There is also a convenience macro wrapper for that crate here: https://github.com/kardeiz/objekt-clonable, which ends up with minimal extra code and just adding supertraits, such as:

#[clonable]
pub trait Recipient: Clone {
 ...
}

Would also have to derive clone on some internal structs that use it too, Stanza, Header, BaseDecryptor, etc.

Could whip up a draft PR if this is something you be interested in. Thanks.