[CMAKE] Install executables

Summary: This diff adds the rules to install the built executables.

Test Plan:
For all platforms:

mkdir -p buildcmake/install && cd install
cmake -GNinja .. -DCMAKE_INSTALL_PREFIX="install" # Other options

depending on the platform

ninja
ninja install

Check that the executables are installed under install/bin/.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4146