Restore support for dynamic PlanetModel in Geo3D [LUCENE-9150]

#8128 removed dynamic planet model support in Geo3D. This was logical at the time (given the state of Lucene and spatial projections and coordinate reference systems). Since then, however, there have been a lot of new developments within the OGC community around [Coordinate Reference Systems](https://docs.opengeospatial.org/as/18-005r4/18-005r4.html), [Dynamic Coordinate Reference Systems](http://docs.opengeospatial.org/DRAFTS/18-058.html), and [Updated ISO Standards](https://www.iso.org/obp/ui/#iso:std:iso:19111:ed-3:v1:en).

It would be useful for Geo3D (and eventually LatLon\*) to support different geographic datums to make lucene a viable option for indexing/searching in different spatial reference systems (e.g., more accurately computing query shape relations to BKD's internal nodes using datum consistent with the spatial projection). This would also provide an alternative to other limitations of the `LatLon*/XY*` implementation (e.g., pole/dateline crossing, quantization of small polygons).

I'd like to propose keeping the current WGS84 static datum as the default for Geo3D but adding back the constructors to accept custom planet models. Perhaps this could be listed as an "expert" API feature?

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Migrated from [LUCENE-9150](https://issues.apache.org/jira/browse/LUCENE-9150) by Nick Knize (@nknize), updated Mar 03 2020

Start by reviewing the Geo3D changes in #8128 and the current PlanetModel-related API. Compare the existing WGS84 default with the requested custom planet model constructors and consider the linked OGC and ISO coordinate reference system material. Done means Geo3D again accepts custom planet models while retaining WGS84 as the default.

Written by the indexing model from the issue text.