[reginaldalfret]

Description of changes

This change ensures that a custom logger supplied via the programmatic WithLogger option is preserved when CLI flags or configuration files specify --structured-logs or --quiet.

1. Preserve custom logger:

   • Previously in loadConfig, c.logger was unconditionally re-assigned if c.conf.StructuredLogs or c.conf.Quiet was true, overwriting any custom cloudsql.Logger passed by the caller in WithLogger.
   • Added a customLogger indicator to Command set by WithLogger so that custom loggers take precedence over default logger instantiation.

2. Add WithStructuredLogging() Option:

   • Added WithStructuredLogging() to cmd/options.go alongside WithQuietLogging() and WithDebugLogging(), allowing programmatic enablement of structured JSON logging.

3. Regression Tests:

   • Added WithStructuredLogging test case to TestCommandOptions.
   • Added TestCommandOptionsOverridesCLI test cases verifying that WithLogger takes precedence when --structured-logs or --quiet flags are supplied.