der: use Cow for *Ref types

[dishmaker]

Following https://github.com/RustCrypto/formats/pull/1778#issuecomment-2840198645, we need to investigate why Cow<*Ref> does not work.

One major issue is that Clone is implemented on Ref types. https://github.com/RustCrypto/formats/blob/217b6eff7bf1b15d4fea21a3a186c2faee4a4344/der/src/asn1/ia5_string.rs#L37-L40

which collides with (if i remember correctly): https://doc.rust-lang.org/std/borrow/trait.ToOwned.html#impl-ToOwned-for-T