PROJ pipeline generator: recognize opposite Helmert transformations using a different convention

[rouault]

This helps for example for
projinfo -s EPSG:4284 -t "+proj=tmerc +lat_0=0 +lon_0=63 +k=1 +x_0=11500000 +y_0=0 +ellps=krass +towgs84=25,-141,-78.5,0,0.35,0.736,0 +units=m +no_defs +type=crs"

where we previously generated the following pipeline:

+proj=pipeline
  +step +proj=axisswap +order=2,1
  +step +proj=unitconvert +xy_in=deg +xy_out=rad
  +step +proj=push +v_3
  +step +proj=cart +ellps=krass
  +step +proj=helmert +x=25 +y=-141 +z=-78.5 +rx=0 +ry=-0.35 +rz=-0.736 +s=0
        +convention=coordinate_frame
  +step +inv +proj=helmert +x=25 +y=-141 +z=-78.5 +rx=0 +ry=0.35 +rz=0.736 +s=0
        +convention=position_vector
  +step +inv +proj=cart +ellps=krass
  +step +proj=pop +v_3
  +step +proj=tmerc +lat_0=0 +lon_0=63 +k=1 +x_0=11500000 +y_0=0 +ellps=krass

With that optimization, it reduces to:

+proj=pipeline
  +step +proj=axisswap +order=2,1
  +step +proj=unitconvert +xy_in=deg +xy_out=rad
  +step +proj=tmerc +lat_0=0 +lon_0=63 +k=1 +x_0=11500000 +y_0=0 +ellps=krass

Related to https://lists.osgeo.org/pipermail/proj/2022-July/010671.html