migrate: TxDescriptor<
    { weight_limit: { proof_size: bigint; ref_time: bigint } },
>

When a migration is in progress, this dispatchable can be used to run migration steps. Calls that contribute to advancing the migration have their fees waived, as it's helpful for the chain. Note that while the migration is in progress, the pallet will also leverage the on_idle hooks to run migration steps.