[Chronik] Move BUILD_BITCOIN_CHRONIK_PLUGINS so we can access it from functional tests

Summary:
By moving the compilation flag to /CMakeLists.txt, we can use it in config.ini.cmake.in and use ENABLE_CHRONIK_PLUGINS in functional tests.

chronik_plugins_setup.py tests just that.

Test Plan:

cmake .. -GNinja -DBUILD_BITCOIN_CHRONIK=1 -DBUILD_BITCOIN_CHRONIK_PLUGINS=1
ninja && ./test/functional/test_runner.py chronik_plugins_setup

The test is correctly skipped without -DBUILD_BITCOIN_CHRONIK_PLUGINS=1

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15394