Add the `pushUrl` option for `htmx.ajax()` calls

Currently, there's no way to configure `hx-push-url` programatically using `htmx.ajax()`. It would be useful to have a `pushUrl` option there as well.

As far as I know, the only solutions are sending a `'HX-Push-Url'` header in the response or using `{ source: element }` where the referenced element has a `hx-push-url` attribute. The first is hard to maintain, and the second is quite hackish.