Add cmake package config

[securitykernel]

Adds a CMake package configuration for the library, so users of Botan using CMake to link against Botan don't need to invoke pkg-config or write a CMake find module (rnpgp/rnp#2121). Unlike suggested in #3490, we don't write a find module, as package configuration files now seem to be the recommended way. The package configuration files are installed with Botan and can then be used as follows:

# find any Botan version
find_package(Botan REQUIRED)

# find a minimum Botan version (>= 3.3.0)
find_package(Botan 3.3.0 REQUIRED)

# should be able to use ranges, too
find_package(Botan 3.0.0...3.3.0 REQUIRED)

# even check if certain modules are installed
find_package(Botan 3.3.0 REQUIRED COMPONENTS rsa ecdsa kyber)

# finally link with Botan
target_link_libraries(my_target PRIVATE Botan::Botan)

Also adds tests to the CI. Tests are provided in src/scripts/ci/cmake_tests using an example CMake project.

Closes #3490.

[coveralls]

coverage: 91.714% (-0.003%) from 91.717% when pulling 3a052c4 on sk/cmake-config into 84a828a on master.

[randombit]

So the CMake goo itself is fine (well I haven't reviewed it but no objections in principle to shipping something like this) but I'd rather the test of the CMake integration not go in tests - I know it is is a test but in my view tests is "the main test suite", which this is not a part of. I think examples would actually be more appropriate since AIUI the CMakeLists.txt currently in src/tests/cmake is exactly an example of how to link to Botan using CMake.

[securitykernel]

Sure, I'll move it. Then this answers the question where to run the tests (with the examples).

[securitykernel]

Shared linking works fine already, static is a bit more complicated to set up, but I would like to give it a try. I'd also like to ask the CMake devs for a review.

[mathstuf]

A pass over this was requested on Slack. Take as you see fit.

[reneme]

@mathstuf Thanks a lot for your review! Your hints were insightful and made this a much better package config for sure.

Regarding relocatablility: We now defined the locations of includes and libraries relative to the botan-config.cmake file. For instance, the include directory is located as ${CMAKE_CURRENT_LIST_DIR}/../../include/botan-%{version_major}. As long as package maintainers don't change the install layout that should be fine. Nevertheless, it still feels rather hacky and brittle. Is there a better way?

Maybe @thesamesam has an opinion or insight about it as well? 😄

[securitykernel]

@randombit I think this is ready now, so I removed the draft description. I also added a docstring and removed one leftover configure.py cmake foo.

[securitykernel]

Squashed and rebased now.

[randombit]

Did not review the CMake configs, changes to Python look fine