Add support for eligibility token in payments

[j027]

This pull request adds support for eligibility tokens when making payments. It also updates the user agent to match the latest version of the venmo mobile app. It seems like the eligibility token isn't strictly needed, but this should align better with the current behavior of the mobile app.

There's probably some formatting stuff I need to clean up, so let me know how I can fix that.

[joshhubert-dsp]

Hey @j027, a year later, is this PR still working for you for sending payments? It definitely gets closer than master for me, and consistently gets me this new error message (which is an improvement). This is after copying most of the header sent from my phone, minus the datadog/traceparent/tracestate bits that vary every request. Maybe those are important, not sure.

HttpCodeError: HTTP Status code is invalid. Could not make the request because -> 400 Bad Request.
Error: {'error': {'message': "Sorry, you can't pay this user right now.", 'code': 92000, 'links': [], 'title': "That didn't work"}}

[j027]

I’ve not touched this in a while, I know it used to work, but it’s probably best to MITM the latest app and see how it behaves. I got my Venmo account suspended when I tried to automate payments, so I kind of gave up on it after that.