Request: Stabilize ZSTD_createDDict_byReference

[pdillinger]

This function (ZSTD_createDDict_byReference) is used by RocksDB, and long has been. It is essentially required for digested dictionary support so that we can manage raw bytes ourselves without memory duplication. It would be good (eventually) not to require ZSTD_STATIC_LINKING_ONLY for this feature. Some context in facebook/rocksdb#13362

[Cyan4973]

Context:
we are planning to release v1.5.7 soon.
Normally, such an updated version number doesn't include a change to the stable user API scope.
So either we do it nonetheless,
or we "properly" update to v1.6.0 instead,
but, in this case, several other topics currently registered for the v1.6.0 release would have to be considered too,
which would add workload, which would delay the release...

We have some tough choice ahead here.

[pdillinger]

This has been quietly outstanding for a while. Just requesting you get to it when you pretty easily can.