Default number of network retries to 2

[brandur]

Currently, it's possible to have the library retry intermittently failed
requests by configuring MaxNetworkRetries on a BackendConfig.
Because this is a pretty useful feature that many users will never
discover because it's off by default, we've been mulling around the idea
internally to change that default on the next major so that more people
get access to it. We're about to release V71, so now is an opportune
moment.

The slight complication is that BackendConfig.MaxNetworkRetries is
currently a simple int, which means that it's hard for us to recognize
an unset value versus an explicitly set 0 (the same problem we had with
fields on parameter structs for years). So by example, this code is
problematic:

if conf.MaxNetworkRetries == 0 {
    backend.MaxNetworkRetries = 2
}

The most obvious solution is that change MaxNetworkRetries to a
nilable pointer, and have it set using our stripe.Int64 helper,
exactly as we do for parameter structs. So compared to today,
configuring it would change like this:

 config := &stripe.BackendConfig{
-    MaxNetworkRetries: 2,
+    MaxNetworkRetries: stripe.Int64(2),
 }

It's not too bad, and follows convention found elsewhere in the library,
but will require a small code update for users.

The slight follow on complication is that to make BackendConfig
self-consistent, I also changed the other primitives on it to also be
pointers, so EnableTelemetry changes from bool to *bool and URL
changes from string to *string. I don't think this is a big deal
because ~99% of users will probably just be using the defaults by having
left them unset.

r? @ob-stripe @remi-stripe A little more complicated than I was
originally hoping, but not too bad. I still think we should do it.
Thoughts?
cc @stripe/api-libraries

---

Note: Targets the branch in #1055 for V71 integration.

[brandur-stripe]

(Also, if you guys think this is okay, I'll update the README. I didn't bother for now.)

[brandur-stripe]

Thanks OB! Rebased on rewrote the appropriate section in the README.