Add plugin for Ping federated authentication

[missing0x00]

Main requirement for the Ping authentication flow is that it needs a new URI and cookie value with each request, otherwise it will start returning "page expired" warnings.

I included a section which will append the "ping-messages" div from the response to ensure accuracy. This can be removed if it's too verbose, most of the messages will just be "We didn't recognize the username or password you entered. Please try again."

[knavesec]

@missing0x00 Just wanting to do some due diligence before merging since I don't have a good way to test this right now.

1. How many targets have you tested this against?
2. Have you tested both positive and negative credentials to verify behavior?
3. How extensively has this been used/trialed?

[missing0x00]

@knavesec Great questions, I would love to test it more extensively as well.

1. One target so far, hoping for more to test.
2. Not exactly. The positive response is based on a manual authentication capture through Burp. The user changed their password shortly after that so I was unable to confirm it with this plugin. I included the secondary check for any response that does not contain the pf.pass field since this is likely a success. This is also part of why I left in the code to retrieve the ping-messages portion of the response.
3. Ran over several days with many guesses, but no successes. Most likely they were all true negatives based on the target's password policy. All of the responses included the expected error message for invalid credentials, with no indication of blocks or lockouts.

[knavesec]

Also update the readme with the new plugin and add yourself to the credits if desired