Jetpack Search: Add option to make search box position "fixed"

#### Is your feature request related to a problem? Please describe.

Jetpack search is a paid feature, and users paying for it will often want to make this search box more prominent. Some of the newer themes (Seedlet and Varia, among others) do not have a sidebar widget area, which will prevent us from adding the search box in a sufficiently prominent place.

#### Describe the solution you'd like

I would propose adding a checkbox to add `position:fixed` to the search widget. This would increase the visibility of the searchbox. The [Cookies & Consents Banner](https://en.support.wordpress.com/cookie-widget/) widget already shows this way.

#### Describe alternatives you've considered

Making a widget sticky might not always work 100% out of the box on any give themes, so we might want to offer a few additional options such as:

- Stick to top or bottom
- Deactivate on mobile
- Set a custom z-index vaule

#### Additional context

Here is a mockup of what it could look like on mobile:


And here is what the feature could look like, on its minimalistic approach.


The issue names Jetpack Search's search widget and points to the Cookies & Consents Banner widget as a comparison; begin by locating those widget implementations. Confirm the intended scope for fixed positioning, including the proposed top/bottom, mobile, and z-index options. Done means the selected positioning behavior works as shown in the supplied mockups.

Written by the indexing model from the issue text.