extend HSETEX with NX option

[romange]

Currently, HSETNX has exactly 3 arguments.
We can add the forth optional TTL in seconds for the field to expire (see also https://www.dragonflydb.io/docs/command-reference/hashes/hsetex).

Update: See the comment below - instead of chaning HSETNX, we can extend HSETEX

[romange]

Actually, HSETEX is considered to be experimental and I think we can still improve/extend it in such that it won't break the existing users.
So https://www.dragonflydb.io/docs/command-reference/hashes/hsetex has the following format:
HSETEX key seconds field value [field value ...]
we can introduce an optional NX like this:
HSETEX key [NX] seconds field value [field value ...]

since the parser expects integer as the second argument, this extension will still be well defined and easily parse-able.