feat(cmake): Use modern FetchContent approaches when available

[LecrisUT]

Description

CMake >= 3.24 supports automatic fallback mechanism of find_package -> FetchContent. This is better both for the upstream maintenance and CI, as well as for downstream packagers.

Suggested changelog entry:

• Introduced PYBIND11_CATCH_TESTS and PYBIND11_EIGEN_TESTS CMake options
• Switched to using find_package -> FetchContent fallback for third-party dependencies when available (CMake >= 3.24). The DOWNLOAD_* options are deprecated in favor of using PYBIND11_*_TESTS and standard FETCH_CONTENT_* options to control this

[LecrisUT]

A few follow-ups that I am interested:

• Dropping the complicated handling of eigen3 find_package stuff
• Dropping the support of the old eigen3 and catch2 versions
• Dropping the various Find* modules