Add digest processor xcm emulator (#7915)

Currently parachains are injecting through the xcm-emulator the default
digests in each block, something that can prevent testing certain
consensus aspects.

We propose to add the type `DigestProvider`, which needs to implement
the trait `Convert<blockNumber, Digest>`. The idea is that we can call
the implementation of this trait before initializing every block, and
thus, allowing us to inject custom digests provided by this trait.

Obviously the default behavior persists if you set this type to `().`

The utilization of the `Convert` trait was arbitrary, as it was the
easiest to accomplish the solution. I am not against using a custom
trait defined for this purpose if that is preferred.

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>