Add support for digest item other type

[fanyang1988]

The DigestItem type in substrate has a other type:

pub enum DigestItemRef<'a> {
        ///...
	/// Any 'non-system' digest item, opaque to the native code.
	Other(&'a [u8]),
	///...
}

This will be used for some chain based on substrate, and if use this digestitem, the smoldot will failed to sync blocks from it.

This PR fix this problem by add this type to DigestItem.

[cla-bot-2021]

User @fanyang1988, please sign the CLA here.

[tomaka]

Thanks for the PR! Just some nit.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────
       +2297 ┊ smoldot::header::DigestItemRef::scale_encoding::habfb102cc6170922
       +2221 ┊ smoldot::header::decode_item::h039dd64e1adad4f3
       +2128 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h00336a7c40714737
       -2128 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::ha37e5cb529741eae
       -2120 ┊ smoldot::header::DigestItemRef::scale_encoding::hc0b51d9a08e6b3a7
       +2049 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::next::h432f3a3748ed5d5e
       -2049 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::next::he340e64beffe3da1
       -1985 ┊ smoldot::header::decode_item::h2698e13b5ccaa48c
       +1924 ┊ <alloc::vec::Vec<T,A> as core::clone::Clone>::clone::hed4e0d859c2ad1b9
       -1872 ┊ <alloc::vec::Vec<T,A> as core::clone::Clone>::clone::hf7efca5ce0a7d0f2
       +1038 ┊ <smoldot::header::DigestItem as core::convert::From<smoldot::header::DigestItemRef>>::from::hd4c13201c3b4cc4e
        -969 ┊ <smoldot::header::DigestItem as core::convert::From<smoldot::header::DigestItemRef>>::from::hac44ddc00bed56dd
        -790 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::h58e79f49ba648a2a
        +790 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::h8b42abfa55cc78e8
        +771 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::h3259c26d93f5b5f7
        -771 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::hc42dc7b2c472e926
        +692 ┊ <smoldot::header::DigestItemRef as core::convert::From<&smoldot::header::DigestItem>>::from::h03d6c788f46ff0d1
        -656 ┊ <smoldot::header::DigestItemRef as core::convert::From<&smoldot::header::DigestItem>>::from::hf3bc34bf6d1adedd
        -648 ┊ smoldot::executor::runtime_host::run::h773fc55c97528118
        +648 ┊ smoldot::executor::runtime_host::run::h914460ce1944f8a5
         +60 ┊ ... and 79 more.
        +630 ┊ Σ [99 Total Rows]