No digest item for a custom consensus engine

[khssnv]

Hello! I work on a Substrate-based chain with a custom consensus engine and as suggested in

try-runtime-cli/core/src/common/empty_block/inherents/providers.rs

Lines 96 to 98 in b45be7d

	/// If it does not work for your Substrate-based chain, [please open an issue](https://github.com/paritytech/try-runtime-cli/issues)
	/// and we will look into supporting it.
	struct SmartInherentProvider {

reporting a case when SmartInherentProvider doesn't work for me.

Currently, providers.rs only accounts for BABE and AURA when constructing the digest for the empty block. In my case for QF Network's blockchain node QuantumFusion-network/qf-solochain we are developing a custom consensus engine called SPIN with its own engine ID SPIN_ENGINE_ID. Because of this there is no slot with the expected engine ID in the empty block and the slot number validation fails during try-runtime usage.

It is easy to add custom engine ID in a fork project like:

// core/src/common/empty_block/inherents/providers.rs#L128
let digest = vec![
    DigestItem::PreRuntime(
        BABE_ENGINE_ID,
        PreDigest::SecondaryPlain(SecondaryPlainPreDigest {
            slot,
            authority_index: 0,
        })
        .encode(),
    ),
    DigestItem::PreRuntime(AURA_ENGINE_ID, slot.encode()),
    DigestItem::PreRuntime(*b"spin", slot.encode()),
];

But perhaps there are some recommendations on how to properly support custom engine IDs in the digest? Thank you.