Add support for the "fron" log item

[tomaka]

Close #2139

I wanted to add a unit test, but we have no good way to obtain a SCALE-encoded header from a Substrate node.
Since the code is just copy-pasted from the one of the Beefy and Polkadot log items, it should work.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────
       +2450 ┊ smoldot::header::DigestItemRef::scale_encoding::h9dc96e920f4b74e6
       -2249 ┊ smoldot::header::DigestItemRef::scale_encoding::h416dbac060fc8afd
       +2144 ┊ smoldot::header::decode_item::h4c68c78889f042ee
       -2131 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h15b5542a4e086d98
       +2131 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h4b69853802ad2f51
       -2080 ┊ smoldot::header::decode_item::h0013c67d9db77ed0
       -2025 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::next::h9f9bf13bb22dfabc
       +2025 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::next::he1942b7180f967df
       +1960 ┊ <alloc::vec::Vec<T,A> as core::clone::Clone>::clone::h9cf93b983b596272
       -1908 ┊ <alloc::vec::Vec<T,A> as core::clone::Clone>::clone::h8b7f13a1cf9d492d
        +969 ┊ <smoldot::header::DigestItem as core::convert::From<smoldot::header::DigestItemRef>>::from::h3898fa60b74410d0
        -790 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::h84fc6d3ba7851f89
        +790 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::hc5a56bb62685d478
        +771 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::h23aee6591bd3d9b2
        -771 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::fold::heac076db79250723
        +659 ┊ <smoldot::header::DigestItemRef as core::convert::From<&smoldot::header::DigestItem>>::from::ha3a2fda7c69f6958
        +658 ┊ smoldot::executor::runtime_host::run::h09cd88be0529e47a
        -658 ┊ smoldot::executor::runtime_host::run::h4b232ef7eda411d3
        -635 ┊ <smoldot::header::DigestItem as core::convert::From<smoldot::header::DigestItemRef>>::from::h1bd41493023ddc6f
        -615 ┊ <smoldot::header::DigestItemRef as core::convert::From<&smoldot::header::DigestItem>>::from::h9d1815cf22624ef9
         +33 ┊ ... and 80 more.
        +728 ┊ Σ [100 Total Rows]

[xcaptain]

Thanks, tested and it works.

[melekes]

👍