Add BestBlockChanged notif in sync and runtime services

[tomaka]

This PR fixes a hole in the notifications sent by the syncing service and runtime service.

The problem is as follows:

• Let's say that that there's a new best relay chain block R1 containing parachain block P1. We report P1 as the new best parachain block.
• Then there's a new best relay chain block R2 containing parachain block P2. We report P2 as the new best parachain block.
• Then there's a new best relay chain block R3 containing parachain block P1.

Normally, we should report P1 as the new best parachain block.
However, it is at the moment not possible to do so. The only way to report a change in the best chain is either when a new block arrives or when a block gets finalized.
This PR therefore introduces a new event BestBlockUpdated, which gets generated specifically in this kind of situation.

In addition to the parachain syncing code, introducing this new event required modifications to several other parts of the code: the JSON-RPC service, the transactions service, the runtime service, and the asynchronous tree.

I took the liberty to rename the non_finalized_headers variable in the JSON-RPC service to headers, as this variable also contains the current finalized block (which is an important detail) and this name is therefore misleading.

I'm aware of the fact that the async_tree is starting to be a bit chaotic, but improving it is a big effort that is out of scope of this PR.

[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.

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 Delta Bytes │ Item
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      +13005 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hc30405b34d49625c
      +11820 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::ha6c9b2ea82f7a6ed
      -11710 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hcc3119285a65f2a2
      -11144 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h7a2403911890797c
       +8779 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h587283da085a7d0b
       -7880 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h9aeb61987864f96f
       +6253 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h5e2b775250716b73
       -5810 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h42450fc13c352bce
       +4643 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h2c065a6672bed1e8
       -4643 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hfc2ac5298e5656dd
       +3109 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hbc3c857cc83c38aa
       -2497 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h8c98ac360a62bd38
       +2437 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h55d3843603f20ca4
       +2275 ┊ smoldot::chain::async_tree::AsyncTree<TNow,TBl,TAsync>::try_advance_output::hc4e84a510243543d
       -2021 ┊ smoldot::chain::async_tree::AsyncTree<TNow,TBl,TAsync>::try_advance_output::h98e053c584885d79
       +1998 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h0bd668b3026bdd5a
       -1998 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h5529772bc8a58636
       -1982 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h659bcc793f2ab8d8
       +1949 ┊ smoldot::chain::async_tree::AsyncTree<TNow,TBl,TAsync>::try_advance_output::h0d369b08bcdc021f
       +1939 ┊ smoldot::chain::async_tree::AsyncTree<TNow,TBl,TAsync>::try_advance_output::he95fbf2689da5d19
        +507 ┊ ... and 619 more.
      +11161 ┊ Σ [639 Total Rows]