feat: add config validate functionality without starting the server

[DavidProdinger]

New CLI-Flag

I have added the CLI flag -validate which checks the configuration and exits before starting the SPOA server.
Example call:

$ coraza-spoa -validate -config /etc/coraza-spoa/coraza.yaml
Loading 1 applications
Configuration is valid

Why?

When modifying rules, there can happen errors like wrong syntax or duplicate ids.
If they are unnoticed and you restart the coraza-spoa it will crash and the WAF isn't handling any requests anymore.
This flag enables you to check the configuration before running into such troubles.

This feature isn't solving the restart without downtime as in #19, but it minimizes the risk of bigger downtimes.

[DavidProdinger]

@fionera What do you think about having there only the short version of the parameters?

The rewrite is already a breaking change, so changing the parameter isn't that big of a deal.
Also having the same syntax as HAProxy makes it easier to remember...

But making the profiling options short, isn't necessary, since they are here for debugging and not for the run in normal operation. I would leave them long.

Suggested change

	flag.StringVar(&configPath, "config", "", "configuration file")
	flag.StringVar(&configPath, "f", "", "configuration file")
	flag.BoolVar(&checkMode, "check", false, "check mode : only check config files and exit")
	flag.BoolVar(&checkMode, "c", false, "check mode : only check config files and exit")
	flag.StringVar(&configPath, "f", "", "configuration file")
	flag.BoolVar(&checkMode, "c", false, "check mode : only check config files and exit")

---

Second consideration (or maybe other PR)
And add a version flag to print it from the binary, it gets it from the VCS at buildtime (since Go 1.18)

flag.BoolVar(&version, "v", false, "print version and exit")

// ...

if version {
    var buildInfo, _ = debug.ReadBuildInfo()
    fmt.Printf("Version: %s", buildInfo.Main.Version)
    os.Exit(0)
}