move files from build to repo root, install header files, install cmake files

[milahu]

mostly cosmetic changes

the build directory should not be part of the source tree

building a project should work like

rm -rf build
mkdir build
cd build
cmake ..
cmake --build .
cmake --install .

cmake should also install

• header files
• cmake files

... so downstream projects can actually use libi2pd

the changes were made with the help of chatGPT

todo

• test the generated cmake files
• remove Makefile* - cmake should work on all platforms

related

• i2pd: init at 2.60.0 conda-forge/staged-recipes#33405
  • build works on linux, macos, windows (azure pipeline)
• Go to file does not list files in directories called "build"

CI errors

• windows msys2 CI build error: undefined reference to std::__cxx11::basic_string #2385
• windows i686 CI build error: target not found: mingw-w64-i686-boost #2386

[orignal]

Let @r4sas review this change.