Add ready-to-use amalgamation to Releases

Is your feature request related to a problem? Please describe.
I use cmake FetchContent to download and build amalgamation automatically. It works fine, but unfortunately makes my build dependent on python. Not big deal but it is kinda frustrating

# Download & build jsoncpp from amalgamation
FetchContent_Declare (
    jsoncpp-dl
    URL https://github.com/open-source-parsers/jsoncpp/archive/refs/tags/1.9.4.tar.gz
    URL_HASH          SHA1=d0cceda7df8d6e77d6acde6b9dea0ff604be6b36
    PATCH_COMMAND python amalgamate.py # Would be nice if I could get rid of this
)
FetchContent_Populate(jsoncpp-dl)
add_library(jsoncpp ${jsoncpp-dl_SOURCE_DIR}/dist/jsoncpp.cpp)
target_include_directories(jsoncpp PUBLIC ${jsoncpp-dl_SOURCE_DIR}/dist/json)
# for sake of brevity, I haven't included code responsible for defining `JSON_DLL` macro

Describe the solution you'd like
Would be nice, if you could just ship something like amalgamation.X.Y.Z.zip with already amalgamated sources, alongside normal source archive. It would simplify deployment

Start by reading amalgamate.py and inspecting how the normal source archive is produced for Releases. Done means each release also publishes an amalgamation.X.Y.Z.zip containing the already-generated sources, so CMake FetchContent can use it without running Python.