cms: feature suggestion: incremental hashing API or allow precomputed digest?

[chenxiaolong]

I noticed that in commit 48fc5ec, support for external signatures was added (thanks for that!). Would it be possible to further extend that API to allow the input to be hashed incrementally, instead of supplying the full Vec<u8>? Or alternatively, can a precomputed digest be supplied? It would be useful in situations where a large amount of data is being signed and it can't be fully read into memory (eg. large firmware images).