This reworks (on proj 6 builds only!) QgsCoordinateTransformContext
to use proj strings of proj coordinate operations to handle the
users' preferred list of operations when transforming coordinates
between two CRSes.

Previously, the context heavily utilised internal transform ID
numbers, which were QGIS specific and relied on matching entries
from the QGIS srs.db file. This approach was undesirable because
it meant QGIS had to maintain and carry it's own table of
possible transform pathways between CRS pairs (which was difficult
to update, impossible to track, and most likely severely out of
date).

Now we can utilse Proj 6's (wonderful!) logic for determining the
best coordinate operation to utilise between two CRSes. All the
old API has been deprecated and no longer works under proj 6, but
that's unavoidable (and unlikely to be in use by plugins anyway,
it's VERY low level stuff).

A further bonus of this work is that QgsCoordinateTransform no
longer relies on proj strings of the source/dest CRS to build
the transform -- the issue with that approach was that proj
strings are lossy (and not always possible to generate), so
now by default we are generating better pathways between CRS
pairs.

This resolves issues with transforms which rely on pivot datums,
such as GDA94 - GDA2020 conversions.

Sponsored by ICSM