Add statistics for assets (or bands)

[m-mohr]

If we provide statistics about minimum and maximum values, the clients can use it to show better default visualizations.

Let's say you have uint8 values (0-255) and your values are only 0-100 the browser stretches from 0-255 by default because uint8 is the only limit the client is aware of. As a result the image gets visualized too dark. If you submit 0-100 as min/max statistics, the image is shown better.