Added within expression filter example

[langsmith]

Resolves #1321 by adding an example of using a filter with the within expression, which allows developers to check whether or not a feature is in the bounds of a GeoJSON polygon. This enables styling point and line data differently if they’re fully within a polygon’s borders.

For now, this example is exactly the same as in the Maps SDK test app mapbox/mapbox-gl-native-android#288

Because we haven't ported Turf buffer (mapbox/mapbox-java#987), it's hard to make more responsive examples with within(). One example would be to draw a new directions route to the map click location, use buffer to calculate the new route's buffer area, and then the POIs would adjust accordingly.

[langsmith]

Note to self/others. within() can be combined with a "circle". Not an actual CircleLayer, but a Polygon geometry with 360 steps: https://docs.mapbox.com/android/java/examples/turf-physical-circle. Would be good to make another example where a circle can be moved by clicking on the map and then new POIs are visible only within the circle.

[mattRippin]

@langsmith Can this be used with VectorSource?

[langsmith]

Yes it should work with a VectorSource @mattRippin

[mattRippin]

Yes it should work with a VectorSource @mattRippin

I logged an issue against it with some sample code and gave you a tag. I can't seem to identify what I'm missing.

Really appreciate the quick response from you btw.