Support ITRF plate motion model transformations with OBS params

[trey-stafford]

ITRF2020 parameters were added in proj 9.5.0. See the PR that added these params here: https://github.com/OSGeo/PROJ/pull/4235

The paper that published the PMM parameters for ITRF 2020 can be found here: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL106373

The paper notes that:

In order to be consistent with the ITRF2020 frame, users of the ITRF2020-PMM should not only use the plate rotation poles listed in Table 1, but also add the translation rates listed in Table 2. However, it is recommended to ignore the artifactual vertical component of the predicted velocities resulting from the addition of the ORB, and to only consider the horizontal components of the predicted velocities.

The params added in the proj PR linked above provide both transformation parameters with and without the ORB components.

E.g., ITRF2020:ANTA gives the transformation params for Antarctica without the ORB and ITRF2020:ANTA_T gives the params with the ORB components.

As mentioned above, the ORB components should be applied for horizontal measurements and not for the vertical component.

Based on this, to properly support PMM transformations for ITRF2020, we would want to support the use of the ITRF2020:ANTA_T for the lat/lon and ITRF2020:ANTA for the elevation component. Currently, the code always just adds the plate name without a _T.

[trey-stafford]

There are also {PLATE}_T params available for ITRF2008: https://github.com/OSGeo/PROJ/blob/master/data/ITRF2008

[trey-stafford]

Some context in this proj mailing list thread about the _T params: https://lists.osgeo.org/pipermail/proj/2022-February/010537.html