Embed metadata inside the tile itself

[nyurik]

In order to simplify MVT->MLT migration, we need to support MLT being a proper superset of MVT - in other words to include all the needed metadata. This can be accomplished with:

• prepending the metadata protobuf to each layer
• introducing a different bit-optimized metadata format, rather than dealing with ProtoBufs (I think this may be better)

Afterwards, we can stabilize adding metadata to the tilejson, preferably parseable with the json schema