# Description

This is a continuation of
https://github.com/paritytech/polkadot-sdk/pull/5666 that finally fixes
https://github.com/paritytech/polkadot-sdk/issues/5333.

This should allow developers to create custom syncing strategies or even
the whole syncing engine if they so desire. It also moved syncing engine
creation and addition of corresponding protocol outside
`build_network_advanced` method, which is something Bastian expressed as
desired in
https://github.com/paritytech/polkadot-sdk/issues/5#issuecomment-1700816458

Here I replaced strategy-specific types and methods in `SyncingStrategy`
trait with generic ones. Specifically `SyncingAction` is now used by all
strategies instead of strategy-specific types with conversions.
`StrategyKey` was an enum with a fixed set of options and now replaced
with an opaque type that strategies create privately and send to upper
layers as an opaque type. Requests and responses are now handled in a
generic way regardless of the strategy, which reduced and simplified
strategy API.

`PolkadotSyncingStrategy` now lives in its dedicated module (had to edit
.gitignore for this) like other strategies.

`build_network_advanced` takes generic `SyncingService` as an argument
alongside with a few other low-level types (that can probably be
extracted in the future as well) without any notion of specifics of the
way syncing is actually done. All the protocol and tasks are created
outside and not a part of the network anymore. It still adds a bunch of
protocols like for light client and some others that should eventually
be restructured making `build_network_advanced` just building generic
network and not application-specific protocols handling.

## Integration

Just like https://github.com/paritytech/polkadot-sdk/pull/5666
introduced `build_polkadot_syncing_strategy`, this PR introduces
`build_default_block_downloader`, but for convenience and to avoid
typical boilerplate a simpler high-level function
`build_default_syncing_engine` is added that will take care of creating
typical block downloader, syncing strategy and syncing engine, which is
what most users will be using going forward. `build_network` towards the
end of the PR was renamed to `build_network_advanced` and
`build_network`'s API was reverted to
pre-https://github.com/paritytech/polkadot-sdk/pull/5666, so most users
will not see much of a difference during upgrade unless they opt-in to
use new API.

## Review Notes

For `StrategyKey` I was thinking about using something like private type
and then storing `TypeId` inside instead of a static string in it, let
me know if that would preferred.

The biggest change happened to requests that different strategies make
and how their responses are handled. The most annoying thing here is
that block response decoding, in contrast to all other responses, is
dependent on request. This meant request had to be sent throughout the
system. While originally `Response` was `Vec<u8>`, I didn't want to
re-encode/decode request and response just to fit into that API, so I
ended up with `Box<dyn Any + Send>`. This allows responses to be truly
generic and each strategy will know how to downcast it back to the
concrete type when handling the response.

Import queue refactoring was needed to move `SyncingEngine` construction
out of `build_network` that awkwardly implemented for `SyncingService`,
but due to `&mut self` wasn't usable on `Arc<SyncingService>` for no
good reason. `Arc<SyncingService>` itself is of course useless, but
refactoring to replace it with just `SyncingService` was unfortunately
rejected in https://github.com/paritytech/polkadot-sdk/pull/5454

As usual I recommend to review this PR as a series of commits instead of
as the final diff, it'll make more sense that way.

# Checklist

* [x] My PR includes a detailed description as outlined in the
"Description" and its two subsections above.
* [x] My PR follows the [labeling requirements](

https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md#Process
) of this project (at minimum one label for `T` required)
* External contributors: ask maintainers to put the right label on your
PR.
* [x] I have made corresponding changes to the documentation (if
applicable)