Normalize some of blocks' presentational data

[omotnyk]

Same as #258 but for some of the BlockKit models.

[leonidhladii]

@omotnyk looks good at all.
Do we plan to add the same validation stuff to External...Menu, Date/Time pickers, OverflowMenu?

[omotnyk]

@leonidhladii Yeah, but I didn't want to spend time normalizing all the input data that we don't use at this point.

Still, I realized that we at least use ExternalSelectMenu so I'll add its normalization as well. Thanks!

[omotnyk]

@leonidhladii Getting back to this, I don't think there is much we can normalize in ExternalSelectMenu. There is a placeholder but don't really use it in a way that can cause issues with its length.

Do you think we can merge this PR?

[leonidhladii]

Works for me, good job 🚀