feat: add zot subcommand to enable testing retention policy settings (#3449)

feat: add verify-feature retention subcommand with comprehensive testing and validation

Add a `verify-feature retention` subcommand that allows users to preview and
validate retention policy changes without running the actual Zot server.
The command runs GC and retention tasks in dry-run mode for immediate feedback.

- Run verify-feature retention standalone without starting the server
- Preview retention policy decisions in dry-run mode
- Configurable GC interval override via command-line flag
- Optional timeout for task completion
- Configurable log output (stdout or file)

Basic usage:
```bash
zot verify-feature retention <config-file>
```

With log file output:
```bash
zot verify-feature retention -l /var/log/zot-retention-check.log <config-file>
```

With GC interval override (runs GC tasks every 30 seconds):
```bash
zot verify-feature retention -i 30s <config-file>
```

With timeout (wait up to 5 minutes for tasks to complete):
```bash
zot verify-feature retention -t 5m <config-file>
```

Combined flags:
```bash
zot verify-feature retention -l /var/log/zot-retention-check.log -i 1m -t 10m <config-file>
```

The command supports overriding GC settings from the config:
- `-i, --gc-interval`: Override the GC interval setting (applies to all storage paths including subpaths)

- Refactored `RunGCTasks` from `controller.go` to be reusable
- Added `checkServerRunning` validation to prevent conflicts
- Implemented signal handling for graceful shutdown
- Added configuration sanitization and logging
- Set GCMaxSchedulerDelay programmatically (not user-configurable)

Added tests for coverage on main function:
- Negative test cases (no args, bad config, GC disabled, server running)
- Both BoltDB and Redis
- Retention enabled scenarios with complex image setups
- Retention disabled scenarios
- Delete referrers functionality
- Subpaths configuration
- GC interval override validation

Run the verify-feature retention tests:
```bash
go test -v ./pkg/cli/server -run TestRetentionCheck
```

Signed-off-by: Andrei Aaron <andreifdaaron@gmail.com>

pkg/api/config/config.go

@@ -36,6 +36,10 @@ type StorageConfig struct {
 	Retention     ImageRetention
 	StorageDriver map[string]interface{} `mapstructure:",omitempty"`
 	CacheDriver   map[string]interface{} `mapstructure:",omitempty"`
+
+	// GCMaxSchedulerDelay is the maximum random delay for GC task scheduling
+	// This field is not configurable by the end user
+	GCMaxSchedulerDelay time.Duration `yaml:"-"`
 }
 
 type ImageRetention struct {