Add GeoPoint in StreamInput/StreamOutput

[clement-tourriere]

Fixes transport serialization for geo_point type in multi node environment (possibility to use geo_point in script and fielddata_fields).

Closes #13340

[nik9000]

I'd love to see a test here so it doesn't break in future and so I can be sure exactly what this fixes.

[jpountz]

Another thing that is needed would be for GeoPoint to implement ToXContent so that it is properly rendered in the response. I think otherwise we will use toString() and the current GeoPoint.toString() produces an output that looks like geo-json except that it puts the latitude first :s By using ToXContent at least we could use the non-ambiguous representation that writes geo points as an object with "lat" and "lon" properties.

[javanna]

Another thing that is needed would be for GeoPoint to implement ToXContent

/me wonders why we haven't done it as part of the query-refactoring. sounds like a nice improvement.

One more tiny comment is if we do introduce a public StreamInput#readGeoPoint method, I would also add a corresponding StreamOutput#writeGeoPoint.

[jpountz]

You can ignore my previous comment: I assumed we would use GeoPoint.toString given that GeoPoint does not implement ToXContent but actually we special-case the GeoPoint class in XContentBuilder so everything should be ok.

[s1monw]

merged thanks!