Add a mechanism to register external algorithm, in particular for drivers

[rouault]

Fixes #12101

• GDALAlgorithm: add mechanism to register 'external' algorithms
• GDALDriver: add methods to register algorithms, including deferred loaded drivers. The declaration of the algorithm (just its path) is done in the code integrated in libgdal, but the plugin DLL is loaded only if the algorithm is instantiated
• PDF driver: add 'gdal driver pdf list-layers'
• OpenFileGDB: add 'gdal driver openfilegdb repack'

$ gdal
ERROR 1: gdal: Missing command name.
Usage: gdal <COMMAND> [OPTIONS]
where <COMMAND> is one of:
[...]
  - driver:   Command for driver specific operations.
[...]

$ gdal driver
ERROR 1: driver: Missing subcommand name.
Usage: gdal driver <SUBCOMMAND> [OPTIONS]
where <SUBCOMMAND> is one of:
  - pdf: Command for PDF driver specific operations.
[...]

$ gdal driver pdf list-layers autotest/gdrivers/data/pdf/adobe_style_geospatial.pdf
[
  "New_Data_Frame",
  "New_Data_Frame.Graticule",
  "Layers",
  "Layers.Measured_Grid",
  "Layers.Graticule"
]

$ gdal driver openfilegdb repack poly.gdb && echo yes
yes

$ gdal driver openfilegdb repack poly.gdb --progress
0...10...20...30...40...50...60...70...80...90...100 - done.

[coveralls]

coverage: 70.582% (-0.01%) from 70.593%
when pulling 6f8935c on rouault:plugin_algorithm
into c19543d on OSGeo:master.

[elpaso]

Cool!
This paves the road to useful driver-specific sub commands.

[elpaso]

can we skip this insert call if other.empty() ?

[rouault]

can we skip this insert call if other.empty() ?

we could, but this adds extra code, for little advantage. This isn't a specially performance critical code path, and the overhead of inserting nothing should be un-noticeable