So far, using RSA-PSS, it is caller's responsibility to prepare the input data like:
```
(data (flag pss)
  (hash <algo> <hashed-value>)
  (random-override <salt>))
```
with `<HASHED-VALUE>` (it is the caller who does hashing, beforehand).
It is good to have  "prehash" function (in RFC 8032 (EdDSA) terminology) which hashes input internally when pk_sign-ed.  The data format to use this functionality will be:
```
(data (flags pss prehash)
  (hash-algo <algo>)
  (value <value>))
```