Add the pushUrl option for htmx.ajax() calls

[wout]

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.

[ALameLlama]

I've needed to do this to "fake" hx-boost on elements that aren't forms/a tags, would be nice to have support for this directly using htmx.ajax()