Support cleaner pagination for V1 in stripe.Client

[mbroshi-stripe]

Why?

For V2 support, we offer pagination using Go's iter.Seq2 interface, so iterating over V2 accounts looks like this:

for acct, err := range sc.V2Accounts.All(params) {
        // do something
}

This compares to V1 where it looks like this:

i := sc.Accounts.List(params)
for i.Next() {
	acct := i.Account()
        // do something
}

if err := i.Err(); err != nil {
	// handle
}

With the new stripe.Client type in a future PR, we will be providing the former style pagination for both V1 and V2.

This PR paves the way by introducing the mechanism for V1 to cleanly paginate in List methods.

What?

• Adds an unexported (i.e., internal-only) type v1List with an All method for pagination. This is not currently used anywhere, but will be leveraged by stripe.Client in a subsequent PR.

See Also