Implement support for in-toto sublayouts

[radu-matei]

Original thread:

@radu-matei: Do we want to handle sublayouts at this point in the implementation?
@trishankatdatadog: IDK, maybe, I haven't seen anyone actually use it in practice. Thoughts, @SantiagoTorres and @adityasaky?
@SantiagoTorres: It's used in practice as far as I'm aware, but I think it can be added at a later date without having to reimplement anything