Move vector tile implementation to its own module

[iverase]

This PR moves all new vector tile code into its own pack module called vector-tile. I am removing lots of unused code which was developed as part of the original prototype so the code is looking more similar to the final version. In particular I have removed all the aggregations implementations as we are not using them.

In addition I added a couple of nice things:

• The API now handles cancellation which it turned to be a big performance booster.
• The API supports defining a sort field.
• RestVectorTileAction has been refactor to one class again. I extracted the VectorTileRequest class so it can be unit tested.

[elasticmachine]

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

[imotov]

LGTM