Replace Requestor with StripeClient and HttpClient

[ob-stripe]

Alright, this should be the last of the big changes to the internal requestor logic.

The Stripe.Infrastructure.Requestor static class is replaced by several new concepts:

• IStripeClient is the interface for a Stripe client class. Its contract defines a single method MakeRequestAsync<T>, which accepts a request's arguments (HTTP method, path, options class containing the request's parameters and request options class containing the request's special modifiers) and returns an instance of T (where T is a type that implements IStripeEntity)
• StripeClient is the concrete class that implements IStripeClient. It accepts an optional HttpClient in its constructor. StripeClient delegates HTTP communications to the HttpClient instance and only takes care of decoding the response from Stripe's API
• IHttpClient is the interface for HTTP clients. Its contract defines a single method MakeRequestAsync which accepts a Request and returns a Response
• SystemNetHttpClient is the default concrete implementation that implements IHttpClient. It accepts an optional System.Net.HttpClient instance, or initializes one with default parameters.

This might seem over-engineered but it offers a clear separation of concepts and makes the library easier to test and mock, both by ourselves and by users. (This design is very similar to Twilio's C# library.)

Also, I've renamed the new Request class I added in #1508 to StripeRequest to make it consistent with StripeResponse. I've also extended StripeResponse to contain ~all information about the response and added some helpers to access custom Stripe headers.

Breaking changes:
HttpMessageHandler and Timeout were removed from StripeConfiguration and replaced by StripeClient. Users who want to pass a custom message handler can still do so by passing a custom client. This is a bit more complex, but it's a lot more powerful:

StripeConfiguration.StripeClient = new StripeClient(new HttpClient(new System.Net.HttpClient(messageHandler)));

[ob-stripe]

r? @remi-stripe @brandur-stripe

Whew! I think this is finally ready for review.

[remi-stripe]

minor comments while skimming but deferring the real review to Brandur here.

[brandur-stripe]

Left one comment below, but this looks great — I'm really liking these clean layers!

ptal @ob-stripe

[brandur-stripe]

Just wanted to echo again that I love how short + succinct these sorts of tests come out! Very readable.

[ob-stripe]

I've removed the unsafe properties and changed HttpClient from an abstract class to an interface (and renamed it to IHttpClient).

ptal @brandur-stripe

[brandur-stripe]

I've removed the unsafe properties and changed HttpClient from an abstract class to an interface (and renamed it to IHttpClient).

👍

LGTM.