Generate skippable frame API

[senhuang42]

ZSTD_writeSkippableFrame(void* dst, size_t dstCapacity, const void* src, size_t srcSize, U32 magicVariant)

Writes a skippable frame into dst buffer, using src content, and using one of the 16 available skippable frame magic numbers, controllable with magicVariant. More detailed documentation in zstd.h.

Test Plan:

• Modify existing unit test to ensure roundtrip-ability

[terrelln]

This looks good to me. Is there a particular use case that motivates adding this to the API?

[senhuang42]

Was mentioned in #2275 that this might be a nice thing to have.

[Cyan4973]

U32 magicVariant

In the future, if we ever come to settle on a few skippable frame IDs implying something meaningful, we could use an enum instead. Or in complementarity, as I think this is compatible with U32, so it should be fine.

Speaking of "meaningful" skippable frame IDs, maybe we could consider those from pzstd, and maybe the seekable format.