Add digest processor xcm emulator

[girazoki]

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.