Add a way to get searchParams in Server Actions

[ptvty]

Related: #50156
Looking at the Network tab I can see that the POST request sent to the server action on form submit already includes all the search params from the current URL, but I couldn't find a way to get them from within the server action function.

There are valid use cases where this is needed.

I'm using a server action as the form action in a pure server component.
The URL already includes multiple search params and I need these params in addition to the form data.
So currently for the server action to have these params I had to get searchParams in the server component, and pass them using an <input type="hidden" /> so I can get those in the handler function as the form data.

This is obviously not efficient as the same data is present in both the URL and the form data!