Added `keys` command

[crodas]

Add keys-related commands

Commands:

• [x] COPY
• [x] DEL
• [ ] DUMP
• [x] EXISTS
• [x] EXPIRE
• [x] EXPIREAT
• [x] EXPIRETIME
• [x] KEYS
• [ ] MIGRATE
• [x] MOVE
• [ ] OBJECT ENCODING
• [ ] OBJECT FREQ
• [x] OBJECT HELP
• [ ] OBJECT IDLETIME
• [x] OBJECT REFCOUNT
• [x] PERSIST
• [x] PEXPIRE
• [x] PEXPIREAT
• [x] PEXPIRETIME
• [x] PTTL
• [ ] RANDOMKEY
• [x] RENAME
• [x] RENAMENX
• [ ] RESTORE
• [x] SCAN
• [ ] SORT
• [ ] SORT_RO
• [ ] TOUCH
• [x] TTL
• [x] TYPE
• [x] UNLINK
• [ ] WAIT

[crodas]

Other commands are going to be taken care on their own PRs.