feat(cmd): Add CAS/CAD commands

[ndyakov]

Adding CAS/CAD commands available in redis 8.4:

SET key value [NX | XX
  | IFEQ match-value | IFNE match-value
  | IFDEQ match-digest | IFDNE match-digest] [GET]
# return changes:
# - if GET specified, returns previous value (regardless of IFEQ etc)
# - if GET not specified, nil reply if operation aborted due to IFEQ etc
DELEX key [IFEQ match-value | IFNE match-value
  | IFDEQ match-digest | IFDNE match-digest]
# returns integer of the number of keys that were removed (so: 0 or 1)
DIGEST key
# returns hex of the specified key (pending "null if not exists?")

I did change the methods and added *Get methods for each SetIF* method, since the responses mean different things (i.e. Set will return status OK but if it is set ... ifneq .. get it still can return OK if the previous value of the key was OK. This way it is more clear, the returned commands are StatusCmd or StringCmd and they both mean different things (should) to the user.
What is still a StatusCmd that can actually hold the previous value, is the return of SetArgs if we have Get=True in the args. Any feedback on how to handle this better is welcome.