Add API Keys Validation

[anikanodia]

Hi, I think it would be pretty cool to have a -validate flag that checks all configured tokens and reports any that are expired/invalid. Would be very cool if subfinder can do this instead of manually having to verify this from time to time.

Thanks for considering it!

[dogancanbakir]

That's a good idea! However, remember that not all APIs provide such validation endpoints. Alternatively, you could fetch the data first and then validate it, which might increase subs usage. Also, we return runtime errors if any source fails or doesn't work, which could be helpful in your situation. What do you think?