[NEW] add 'count' option to the 'RANDOMKEY' command

[hwware]

Regarding to redis#11915

Requirement:

Now RANDOMKEY command return a random key from the currently selected database OR nil when the database is empty
We want to enhancement this command for more features:

1. Add count option to specified number of keys
2. Add duplicate option to allow return duplicated or non-duplicated random key
3. Add pattern option to allow client to specified the key pattern.

The command format could be like this:

RANDOMKEY [COUNT count] [Duplicated ] [Pattern XXXX]
Default: count = 1; non-duplicated; and no patten which means must match exactly keyname