Utils Method for signature verification

Maybe we shall consider adding utils method for handling signature verification.

```
require 'openssl'

  def verify_webhook
    calculated_hmac = OpenSSL::HMAC.hexdigest('sha256', SECRET, request.body.read)
    ActiveSupport::SecurityUtils.secure_compare(calculated_hmac, request.headers["X-Voucherify-Signature"])
  end
```