initial skeleton of vector tiles prototype implementation

[iverase]

Initial commit of a new end point that generates vector tiles from geo spatial data stored in Elasticsearch. It adds three packages in the spatial module:

• org.elasticsearch.xpack.spatial.vectortile: Contains the classes that introduces the new end point.
• org.elasticsearch.xpack.spatial.vectortile.collector: contains different implementation of how to construct the vector tile.
  - org.elasticsearch.xpack.spatial.proto: Google protobuffer classes for generating vector tiles.

The new end-point can be used by calling:

GET {index}/_mvt/{geo_field}/{z}/{x}/{y}

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[imotov]

Let's suppress the broken test and merge this into the feature branch. There are several things that we should fix, but it will be easier to work no it in the branch.

[iverase]

@elasticmachine update branch

[iverase]

@elasticmachine update branch