introduce streaming access to storage items

[TychoVrahe]

introducing access to slices of data from storage.

as preparation for #2521 , where it is needed for accessing homescreen image in memory constraint environment.

[prusnak]

IIRC homescreen is stored as unencrypted value in the storage, no?

If that is the case, I propose to allow streaming only for the unencrypted values. It does not feel right to allow streaming for the encrypted ones.

[TychoVrahe]

Homescreen is indeed unencrypted, i wanted to keep the API universal but its not strictly needed at this point.

just to be clear what this does: basically you can request just part of the stored data, at given offset with specified length. Encrypted items are decrypted at each separate request and just the requested part is copied into the output buffer. IMO this should be fine from security point of view, but it is some added complexity.

[prusnak]

Let's not add features we have no use case for. Especially when it comes to the storage.