Add Point property for mapping point datatype

[russcam]

Relates: #4718, elastic/elasticsearch#53804

This commit adds a Point property to map the new point data type.
A CartesianPoint type is introduced to model points.

[russcam]

The new point datatype can be used in shape queries. Shape queries however are bound to IGeoShape

elasticsearch-net/src/Nest/QueryDsl/Specialized/Shape/IShapeQuery.cs

Lines 34 to 38 in f1ffbf2

	/// <summary>
	/// The geo shape to search with
	/// </summary>
	[DataMember(Name ="shape")]
	IGeoShape Shape { get; set; }

which CartesianPoint does not implement. Think we may need to introduce new shape types here

[russcam]

After conversation with the geo team, it's clarified some questions around CartesianPoint:

CartesianPoint can be used for the null_value of a Point property field mapping, and allow users to use it on their documents to index into the Point field. To query on a Point field using the shape query, the PointGeoShape type can be used.

Another type (CartesianPoint) needs to be introduced because of the formats that must be supported for both the null_value of Point field mappings, and also for formats that may be used on _source documents outside of the client that the client must be able to deserialize:

• Object: { "x": 41.12, "y": -71.34 }
• String: "41.12,-71.34"
• Array: [41.12,-71.34]
• WKT: POINT (41.12 -71.34)

[russcam]

This PR has been rebased against master and is ready for review, @Mpdreamz.

I think we should seriously consider renaming IGeoShape to IGeometry in 8.x, and the shape types that implement IGeoShape to remove the Geo name part. The renames better reflect their intent and usage, though I do appreciate that such a change could be a reasonably large breaking change, as geo queries are popular.

[russcam]

Merging this one in; last test run was green and this test run fails with tests unrelated to the changes.