feat(block_volume): support migration from instance volume

[Codelax]

Provide a solution to easily migrate from instance_volume to block_volume.
This is done in 2 steps:

Starting from your terraform config with your instance_volume

resource scaleway_instance_volume volume {
	type = "b_ssd"
	size_in_gb = 20
}

You can use the new field migrate_to_sbs on your instance's volume to prevent it from synced, thus preventing terraform from detecting that the volume does not exist after its migration. If detected as deleted, terraform would trigger back the creation of the volume.

resource scaleway_instance_volume volume {
	type = "b_ssd"
	size_in_gb = 20
	migrate_to_sbs = true
}

resource scaleway_block_volume volume {
	iops = 5000
	instance_volume_id = scaleway_instance_volume.volume.id
	size_in_gb = 20
}

When doing migration, the new resource block_volume will have the same ID as the old instance_volume. If you replace your usage of scaleway_instance_volume.volume.id to scaleway_block_volume.volume.id, it should not trigger any changes.

resource scaleway_block_volume volume {
	iops = 5000
	size_in_gb = 20
}

The last step is to remove the usage of scaleway_instance_volume, it required multiple steps but you should have migrated to the new SBS api.

The goal of this steps is to provide an easy migration plan to use the new SBS api for your old volumes.
Without this migration plan, any user would need to do a manual migration an tinker with terraform's state