Add Newtonsoft.Json version to X-Stripe-Client-User-Agent

[ob-stripe]

r? @remi-stripe
cc @stripe/api-libraries

Add Newtonsoft.Json version to X-Stripe-Client-User-Agent header.

Unfortunately AFAICT Newtonsoft.Json does not expose its version number, so I had to resort to some fugly reflection.

I've verified this works by spying on the requests:

X-Stripe-Client-User-Agent: {"bindings_version":"37.15.0","lang":".net","publisher":"stripe","stripe_net_target_framework":"net45","lang_version":"Mono 6.4.0.198 (2019-06/fe64a4765e6 Wed)","os_version":"Unix 19.4.0.0","newtonsoft_json_version":"9.0.1.19813"}

[remi-stripe]

That looks good to me and is consistent with other parts of the code. Curious how various frameworks react to this but as long as it can't break anyone that seems totally viable