[drake_cmake_external] Sync external dependency upgrades with Drake

[jwnimmer-tri]

See #385 for the backstory.

Our drake_cmake_external example has a CMakeLists.txt file that pins some ExternalProject_Add dependencies:

drake-external-examples/drake_cmake_external/CMakeLists.txt

Line 35 in a68041a

URL https://gitlab.com/libeigen/eigen/-/archive/3.4.0/eigen-3.4.0.tar.gz

drake-external-examples/drake_cmake_external/CMakeLists.txt

Line 68 in a68041a

URL https://github.com/fmtlib/fmt/archive/refs/tags/11.0.2.tar.gz

drake-external-examples/drake_cmake_external/CMakeLists.txt

Line 91 in a68041a

URL https://github.com/gabime/spdlog/archive/refs/tags/v1.15.0.tar.gz

---

The invariant we want is that those CMake pins match Drake's MODULE.bazel pins:

https://github.com/RobotLocomotion/drake/blob/f481ea4b0bb134508899e3ba7e37bedcb3ad7fc8/MODULE.bazel#L58-L60

bazel_dep(name = "eigen", version = "3.4.0.bcr.3", repo_name = "module_eigen")
bazel_dep(name = "fmt", version = "11.0.2.bcr.1", repo_name = "module_fmt")
bazel_dep(name = "spdlog", version = "1.15.0.bcr.4", repo_name = "module_spdlog")

I am ok matching either Drake's master branch pins, or the pins from Drake's latest stable release.

The goal here is to keep them in sync over time. That might simply mean adding documentation to a playbook somewhere, or adding a new DEE/private/test linter to cross-check, or adding a 'bot script to automate it.

[tyler-yankee]

From f2f, and cross-posting from the above PRs, there are two (really, three) outstanding items here for future improvements:

1. Have the script commit its results, similar to Drake's new_release tooling
2. (a) Add capability to pull the matching version number from Drake's MODULE.bazel, instead of having to be copied manually. (b) Add a linting routine to CI which uses this feature to check for non-matching versions (serving as a ping to upgrade).