Add Belgian geoid model hBG18 to grid alternatives

[AlexBass05]

Corresponding PR in PROJ-data: OSGeo/PROJ-data#77

Created as a draft PR as it requires an EPSG database update to include this change request: https://epsg.org/closed-change-request/browse/id/2022.007

[rouault]

I've rebased your PR on top of #3051 which updates to EPSG v10.054

Things seem to work fine:

$ echo "50 5 0" | PROJ_NETWORK=ON PROJ_DEBUG=2 PROJ_LIB=data bin/cs2cs EPSG:4937 EPSG:9907
[...]
Using https://cdn.proj.org/be_ign_hBG18.tif
50.00	5.00 -44.32

[kbevers]

@rouault is this (and the corresponding PROJ-data PR) ready to merge? I'm gearing up for the release candidate and want to merge as much as possible. If possible I'd like to do the RC tomorrow afternoon (DK time).

[rouault]

merged

[AlexBass05]

I've rebased your PR on top of #3051 which updates to EPSG v10.054

Things seem to work fine:

$ echo "50 5 0" | PROJ_NETWORK=ON PROJ_DEBUG=2 PROJ_LIB=data bin/cs2cs EPSG:4937 EPSG:9907
[...]
Using https://cdn.proj.org/be_ign_hBG18.tif
50.00	5.00 -44.32

Excellent! Thanks a lot for the rebase and the merge.
Testing on my side also indicates that things work fine.