Add ability to allow setting proxy

[RPM1984]

Hi there,

I'm trying to capture the outgoing HelloSign API calls from my .NET Core 3.1 app, but nothing is showing up in Fiddler.

This is because in order for RestSharp calls to show up in .NET Core apps, you need to explicitly set the .Proxy, like so:

var client = new RestClient("https://someapi.com");
client.Proxy = new WebProxy("127.0.0.1", 8888); // IP, Port.

Can i suggest we update the Client constructor to allow this (take in an optional WebProxy parameter)? As right now, it's basically impossible to debug this lib with Fiddler, making it difficult to understand what's going on under the hood and talking with HelloSign support.

I have already tested and confirmed this change works, by building the source from this repository and making the above change, so I'm happy to submit a PR for this, just let me know.

Thanks