add mlt support

[tasiotas]

Please add mlt tile format support.
Seems like maplibre-gl-native already supports it, so not sure where does it fail.

When I try to use it, it just returns blank raster, no errors.

Thank you

[acalcutt]

It is something I would like to see integrated, but it will need some work. The main issue is because tileserver-gl uses a custom request handler, we can't use the built in maplibre-native support for mlt. we would have to handle it on our own in javascript, similar to how we added pmtiles support using the pmtiles adapter in this section of code https://github.com/maptiler/tileserver-gl/blob/master/src/serve_rendered.js#L1375-L1580

We would likely just be able to pull in the source type out of the metadata from the source, and use that to decide how to present the file with the javascript version of mlt, but that would need some code to support that.