feat(cmd): implement CONFIG GET and CONFIG SET

[cankush625]

Implementation

• Added db/config.go — thread-safe Config struct using sync.RWMutex with Get and Set methods. Package-level ServerConfig instance holds defaults (hz=1, active-expire-enabled=yes, maxmemory=0).
• Added db/config_helpers.go — type conversion helpers (parseInt, boolToYesNo, etc.) kept separate to avoid cluttering config.go.
• Updated db/expiry.go — active expiry goroutine now reads CleanupInterval() from ServerConfig on every tick instead of a hardcoded constant. This means CONFIG SET hz takes effect without a server restart.
• Added cmd/config.go — ConfigCmd handler dispatching to configGet / configSet. Unknown subcommands and unknown parameters return specific error messages.
• Added common/constants.go — ConfigCmd constant for the command name.
• Added exc/command_executor.go — wired CONFIG into the executor switch.
• Added docs/config.md and updated docs/README.md and root README.md.

Issue

#40

[cankush625]

🚀