feat(exchanges): add binding for @urql/retry-exchange

[parkerziegler]

This PR adds support for using @urql/retry-exchange.

The binding for this is fairly thin, although we did need to introduce a makeRetryExchangeOptions function to support the creation of a retryExchangeOptions record. There are generally two mechanisms for doing this type of "object creation with potentially undefined values" workflow. The first is what we do here, where we have a function that uses optional labelled arguments to generate a record. For example, calling makeRetryExchaneOptions with no arguments (just the positional unit parameter):

open ReasonUrql

let retryExchangeOptions = Client.Exchanges.makeRetryExchangeOptions();

results in the following object on the JS-side:

{
  initialDelayMs: undefined,
  maxDelayMs: undefined,
  maxNumberAttempts: undefined,
  randomDelay: undefined,
  retryIf: undefined,
}

while calling it with some arguments:

open ReasonUrql

let retryExchangeOptions =
  Client.Exchanges.makeRetryExchangeOptions(~initialDelayMs=2000, ~randomDelay=false, ())

results in the following object on the JS-side:

{
  initialDelayMs: 2000,
  maxDelayMs: undefined,
  maxNumberAttempts: undefined,
  randomDelay: false,
  retryIf: undefined,
}

We could also use an abstract record, but these are generally recommended against because of the ergonomics. The one case where they're useful is when compiling an unspecified property explicitly to undefined would result in negative behavior. In this case, urql checks the truthiness of all properties and supplies defaults downstream, so we should be safe explicitly compiling to undefined!

[kitten]

I'm a bit concerned about adding more snapshots that save the Client's shape 🤔 other than that LGTM of course ✌️

[parkerziegler]

Good shout @kitten. I haven't figured out a good way of testing these new exchanges TBH. I think the tests of "can they be applied" actually aren't meaningful, because even a Client snapshot doesn't verify that the exchange is being applied. We'd need more sophisticated logic to execute requests with a Client instance and spy on some behavior of the exchange. That's more effort than I want to invest just yet, but I'll remove the unnecessary snapshots here 👍