Introduce new Request class to represent requests to Stripe's API

[ob-stripe]

r? @remi-stripe @brandur-stripe

This one is fairly straightforward. It adds a new Stripe.Infrastructure.Request class whose job is to turn the request parameters provided by services:

• HttpMethod: HTTP method (...)
• path: relative path, e.g. /v1/charges
• options: BaseOptions instance containing the request parameters, or null
• requestOptions: RequestOptions instance containing the request's "modifiers" to e.g override the global API key or set an idempotency key

into .NET's HTTP library classes:

• HttpMethod: this one is untouched
• Uri: System.Net.Uri instance containing the full URL, including the base path, and for non-POST requests, the parameters encoded in the query string
• AuthorizationHeader: System.Net.Http.Headers.AuthenticationHeaderValue representing the Authorization header with our preferred "Bearer" scheme and the API key
• StripeHeaders: a dictionary containing Stripe-specific HTTP headers. All requests include Stripe-Version, and may include Stripe-Account and/or Idempotency-Key
• Content: for POST requests, a System.Net.Http.HttpContent instance representing the request's body. For non-POST requests, null.

[brandur-stripe]

Nice!

LGTM.