Add a JLD2 extension to allow serializing geometries

[asinghvi17]

Currently, this just overrides the JLD2 API to serialize and deserialize to/from WKB.

In future the framework is there to ship a CRS along, but we need the supporting API in ArchGDAL to set the spatial reference of a geometry, which has to be wrapped from GDAL still. Would also have to serialize a spatial reference, probably as WKT.

MWE is in the tests. It could potentially use more tests though...

Fixes #445

[yeesian]

The failures are unrelated and I'm okay to add more tests as needed after merging this

[yeesian]

It looks like we're getting the following failure in the tests (for julia 1.6):

JLD2 serialization: Error During Test at /home/runner/work/ArchGDAL.jl/ArchGDAL.jl/test/test_geometry.jl:1043
  Test threw exception
  Expression: AG.toWKT(geom2) == AG.toWKT(geom)
  GDALError (CE_Failure, code 10):
  	Pointer 'hGeom' is NULL in 'OGR_G_ExportToWkt'.

[evetion]

It looks like we're getting the following failure in the tests (for julia 1.6):

JLD2 serialization: Error During Test at /home/runner/work/ArchGDAL.jl/ArchGDAL.jl/test/test_geometry.jl:1043
  Test threw exception
  Expression: AG.toWKT(geom2) == AG.toWKT(geom)
  GDALError (CE_Failure, code 10):
  	Pointer 'hGeom' is NULL in 'OGR_G_ExportToWkt'.

FWIW, Do we still want to support 1.6? LTS is 1.10 now, and the merged package extension will require 1.9?

[asinghvi17]

The extension probably doesn't trigger on 1.6, so I'm not surprised.