Allow a BoundCRS to use a PROJ string transformation

Related to https://lists.osgeo.org/pipermail/proj/2021-February/010040.html

Given test.wkt with
```
BOUNDCRS[
    SOURCECRS[
        GEOGCRS["unknown",
            DATUM["Unknown based on GRS80 ellipsoid",
                ELLIPSOID["GRS 1980",6378137,298.257222101,
                    LENGTHUNIT["metre",1],
                    ID["EPSG",7019]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8901]],
            CS[ellipsoidal,2],
                AXIS["longitude",east,
                    ORDER[1],
                    ANGLEUNIT["degree",0.0174532925199433,
                        ID["EPSG",9122]]],
                AXIS["latitude",north,
                    ORDER[2],
                    ANGLEUNIT["degree",0.0174532925199433,
                        ID["EPSG",9122]]]]],
    TARGETCRS[
        GEOGCRS["WGS 84",
            DATUM["World Geodetic System 1984",
                ELLIPSOID["WGS 84",6378137,298.257223563,
                    LENGTHUNIT["metre",1]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433]],
            CS[ellipsoidal,2],
                AXIS["latitude",north,
                    ORDER[1],
                    ANGLEUNIT["degree",0.0174532925199433]],
                AXIS["longitude",east,
                    ORDER[2],
                    ANGLEUNIT["degree",0.0174532925199433]],
            ID["EPSG",4326]]],
    ABRIDGEDTRANSFORMATION["Transformation from unknown to WGS84",
        METHOD["PROJ-based operation method: +proj=pipeline +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=axisswap +order=2,1 +step +proj=cart +ellps=GRS80 +step +proj=helmert +convention=coordinate_frame +exact +step +inv +proj=cart +ellps=WGS84 +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=rad +xy_out=deg"]]]
```

``projinfo -s @test.wkt -t "WGS 84" -o PROJ -q``

outputs:
```
+proj=pipeline
  +step +proj=unitconvert +xy_in=deg +xy_out=rad
  +step +proj=axisswap +order=2,1
  +step +proj=cart +ellps=GRS80
  +step +proj=helmert +convention=coordinate_frame +exact
  +step +inv +proj=cart +ellps=WGS84
  +step +proj=axisswap +order=2,1
  +step +proj=unitconvert +xy_in=rad +xy_out=deg
```

1 files changed, 12 insertions, 0 deletions

diff --git a/src/iso19111/operation/singleoperation.cpp b/src/iso19111/operation/singleoperation.cpp
index 0cd7b57a..b32b61cd 100644
--- a/src/iso19111/operation/singleoperation.cpp
+++ b/src/iso19111/operation/singleoperation.cpp
@@ -2144,6 +2144,18 @@ bool SingleOperation::exportToPROJStringGeneric(
         return true;
     }
 
+    const char *prefix = "PROJ-based operation method: ";
+    if (starts_with(method()->nameStr(), prefix)) {
+        auto projString = method()->nameStr().substr(strlen(prefix));
+        try {
+            formatter->ingestPROJString(projString);
+            return true;
+        } catch (const io::ParsingException &e) {
+            throw io::FormattingException(
+                std::string("ingestPROJString() failed: ") + e.what());
+        }
+    }
+
     return false;
 }