add serde feature

[SyedAhkam]

Hi,

I wished to send the ProofNodes to an external source or a ZKVM (SP1) but I am unable to use serde to serialize the structure because the type doesn't implement the Serialize trait.

I even tried to take ownership of the inner hashmap, which goes from Nibble --> Bytes, but turns out that the type Nibble is also missing a Serialize implementation.

Any workarounds / is it considerable adding support for it?

[DaniPopes]

Nibbles implements serde under the feature https://github.com/alloy-rs/nybbles/blob/cb21a5a43282577d57a97de277332606e48cdfcc/src/nibbles.rs#L59

It should also be exposed in this crate too, feel free to to open a PR for it