Add interface for stretchable image

[Chaoba]

<changelog>Add interface and demo for stretchable image.</changelog>

#Resolve #147

[tobrun]

Awesome 🚀

[tobrun]

👀 @mapbox/maps-android

[chloekraw]

@mapbox/maps-android how is the documentation for Style.addImage generated?

Will this PR add stretchX, stretchY, and content as valid options on Style.addImage in our SDK docs?

[langsmith]

@mapbox/maps-android how is the documentation for Style.addImage generated?

@chloekraw , I generate the API reference docs by running make android-javadoc on the command line in this repo's root folder.

The addImage() explanations in https://docs.mapbox.com/android/api/map-sdk/9.1.0/com/mapbox/mapboxsdk/maps/Style.html come from what's actually written in the inline docs as the SDK is worked on in Android Studio. I'm not writing them from scratch after the release is done or anything. make android-javadoc automatically turns the inline docs into the reference doc HTML files. Happy to walk through the process and screen share to show you what I mean.