Support for Banner Ad Preloading

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

Yes. Banner ads can frequently take several seconds to load, which can cause significant visual changes and makes it hard to show banners for a shorter time on intermediate screens. Based on information from the Mobile Ads SDK team, this is policy compliant.

**Describe the solution you'd like**
An option to load a banner ad without displaying. A method to determine if an ad is loaded would also be useful.

**Describe alternatives you've considered**
None.

**Additional context**
I will likely be opening a PR for this in the future, though offer no guarantees.

Review the existing banner-ad API and its platform integration, then verify how the Mobile Ads SDK supports loading an ad without displaying it and reporting its loaded state. Done means callers can preload a banner, determine whether it is loaded, and display it through a documented API with appropriate tests.

Written by the indexing model from the issue text.