casext: support auto-embedding data in descriptors

[cyphar]

With #630 we now support reading embedded data from descriptors, but we do not produce those descriptors ourselves. The simplest solution would be to auto-embed any data that is smaller than some arbitrary size (say, 512B or 1K) but it would be nice if this were configurable. I suspect #629 will be the ideal way of configuring this with the Go API.