feat: Add built-in daemon mode support for standalone mode

[Eshaan-byte]

I hereby agree to the terms of the GreptimeDB CLA.

Related Issues

#7314

Summary of changes:

• Added --daemon / -d CLI flag to greptime standalone start command
• Added daemon: bool config option in StandaloneOptions for config file support
• Implemented daemonization logic using the daemonize crate (v0.5)
• Log redirection: stdout and stderr are redirected to logs/greptime.stdout and logs/greptimedb.stderr in the data_home directory
• Unix-only implementation using #[cfg(unix)] conditional compilation

How does this PR work?

When the --daemon flag is passed or daemon = true is set in the config file:

1. The process creates the necessary log directories
2. Opens log files for stdout/stderr redirection
3. Calls daemonize.start() which:
   • Forks the process
   • Detaches from the controlling terminal (setsid)
   • Changes working directory to data_home
   • Redirects stdout/stderr to the log files
4. The parent process exits, leaving the child running as a daemon

Design decisions:

• No PID file management: Kept simple as suggested by maintainers
• Unix-only: Windows doesn't support daemonization in the same way (would require Windows services)
• Log location: Uses existing logging.dir configuration, defaults to {data_home}/logs/

Example usage:

greptime standalone start --daemon
Config file:
[standalone]
daemon = true

PR Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[evenyag]

Hi, please sign your commits to pass the DCO.
https://github.com/GreptimeTeam/greptimedb/pull/7317/checks?check_run_id=56746535242

[sunng87]

Can we keep a pid file so we can implement greptime standalone stop