Replace zalando/go-keyring with 99designs/keyring

[carole-lavillonniere]

Switches to 99designs/keyring for automatic fallback to encrypted file storage when system keyring is unavailable.
The previous implementation using zalando/go-keyring would fail without a keyring service. This would have required a custom fallback implementation (more code to maintain).
99designs/keyring has low maintenance but is stable (keyring APIs rarely change).

Fallback Strategy

// Try system keyrings first (most secure)
ring, err := keyring.Open(config)
if err != nil {
    // Fall back to encrypted file storage
    config.AllowedBackends = []keyring.BackendType{keyring.FileBackend}
    ring, err = keyring.Open(config)
}

1. First attempt tries default backends (system keyrings preferred)
2. If system keyring fails or is broken, retry with FileBackend -> handles both "not available" and "partially broken" scenarios

Storage Locations

Using os.UserConfigDir() for OS-dependant paths:

• Linux: ~/.config/localstack/
• macOS: ~/Library/Application Support/localstack/
• Windows: %APPDATA%\localstack\

[silv-io]

Looks good to me in general, just some minor comments.

Nice that we can use something with integrated file fallback 🚀