g.extension: add CMake support

[nilason]

Add CMake support for g.extension.

Testing will only work from an installed version of GRASS, because the export target files are only created in the install process. (You may of course install to a non-standard location).

This depends preliminary on #6877 (which is included here), so setting this as draft for the time being.

OSGeo/grass-addons#1592 provides an example of the implementation in grass-addons.

[nilason]

Now, after OSGeo/grass-addons@22f101f, the following Addons can be tested:

• i.hyper
• i.landsat
• i.landsat8.swlst
• i.points.auto
• r.flowaccumulation
• r.hypso
• r.in.vect
• r.skyline
• wx.metadata

[nilason]

Some hints for testers with limited experience with CMake:

cd <grass-source-dir>
cmake -B build -DCMAKE_PREFIX_PATH=<std-lib-prefix> -DCMAKE_INSTALL_PREFIX=<non-std-prefix> <GRASS-compilation-args>...
cmake --build build
cmake --install build

• std-lib-prefix, depending on your system/setup, often /usr. You may possibly need to add any non-standard path to CMake config file directories.
• non-std-prefix, by default probably /usr/local, but can be (almost) anywhere (eg. /tmp)
• GRASS-compilation-args, see Compile_and_Install_With_CMake

Run GRASS with <non-std-prefix>/bin/grass and g.extension

[nilason]

Opened for review!

[nilason]

This updates mainly cmake code. The small changes to g.extension.py was made in a careful way as to not cause regression with existing code. Merging this now to simplify testing.