Add support for CMake's "FetchContent" submodule

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

CMake has a "FetchContent" module that permits adding a dependency on another project.

See:
https://cmake.org/cmake/help/latest/module/FetchContent.html

It would be good to have dependabot scan for these dependencies and update them.

Start with CMake's FetchContent module documentation linked in the issue and trace how Dependabot currently discovers dependencies in CMake projects. Done means FetchContent dependencies are detected and eligible updates can be proposed, with tests covering the supported declarations and update behavior.

Written by the indexing model from the issue text.