fix: fetch DNS public key correctly when there's multiple DKIM sig

[richardliang]

• The current Google DNS fetch script isnt compatible with emails that have >1 DKIM signatures
• In tools.js, the script previously extracted out.Answer[0].data which assumes that the public key is in the first element in the array
• For certain domains such as Venmo and Coinbase, there are 2 DKIMs and so 2 elements in the array response. The valid public key is in that second element
• This PR updates the logic to out.Answer[out.Answer.length - 1].data and got everything to work. Not sure if theres any more edge cases
• For example this is the diff between twitter fetch and venmo or coinbase