[Next Major] prevent autopagination from mutating

[richardm-stripe]

This PR causes .ListAutoPaging and .SearchAutoPaging not to mutate the user-provided options argument.

Before this change, there would be a subtle bug/confusing behavior if you ever re-used the same options across multiple list requests: e.g. suppose there are 4 customers ["a", "b", "c", "d"].

var options = new CustomerListOptions()
{
  Limit = 2
};
var service = new CustomerService();

// GET /v1/customers?limit=2 ["a", "b"]
// GET /v1/customers?starting_after=b&limit=2 ["c", "d"]
foreach (var cus of in service.ListAutoPaging(options)) {
  ...
}

// Would not start at the beginning because `options` was mutated.
// GET /v1/customers?starting_after=b&limit=2 ["c", "d"]
foreach (var cus of in service.ListAutoPaging(options)) {
  ...
}

There's a small chance that a user is dependent on this behavior, e.g. you can imagine somebody doing

foreach (var cus of in service.ListAutoPaging(options)) { ... }

var firstIdOfLastPage = options.StartingAfter;
var didAnyPagination = options.StartingAfter != null;

so out of conservatism I would wait to change this behavior until the next major version but I'm open to persuasion just to ship it.