[CMAKE] Use a cmake template for config.ini

Summary:
This simplifies a lot the generation of the config.ini file with
cmake.

Test Plan:

cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake

Check the extension is set to .exe

cmake -GNinja ..

Check all the options but the fuzzer and bitcoind are ON

cmake -GNinja .. -DBUILD_BITCOIN_CLI=OFF -DBUILD_BITCOIN_WALLET=OFF

-DBUILD_BITCOIN_ZMQ=OFF
Check all the options are OFF

With Clang:

cmake -GNinja .. -DENABLE_SANITIZERS=fuzzer

Check the fuzzer is ON

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5761