[MBM] Create runtime API

[ggwpez]

We should probably add a runtime API to expose information about current MBMs.

There are lots of events to follow what is happening, but for just querying whether a migration is ongoing, it is required to read storage directly.

The RAPI should expose something like this:

trait Mbm {
    /// Whether there are migrations currently running.
    fn ongoing() -> MbmIsOngoing;

    /// The current progress. Returns `Some(_)` iff `ongoing()` is either `Ongoing` or `Stuck`.
    fn progress() -> Option<MbmProgress>;

    /// The prefices that are being migrated and possibly not decodable during this time.
    fn affected_prefices() -> Vec<Vec<u8>>;  // from https://github.com/paritytech/polkadot-sdk/issues/6128
}

enum MbmIsOngoing {
    Ongoing,
    NotOngoing,
    Stuck,
}

struct MbmProgress {
    /// Index of the currently running migration.
    current_migration: u32,

    /// Number of total migrations that will run.
    total_migrations: u32,

    /// Number of steps that the current migration already took.
    current_migration_steps: u32,

    /// Number of steps that the current migration is allowed to take at most.
    current_migration_max_steps: u32,
}

I woul wait with this issue once #6128 is settled.