v5: Split external library configuration out of FindBaselibs.cmake

[mathomp4]

Summary

• Move NetCDF/HDF5/ESMF/FMS dependency-target creation out of FindBaselibs.cmake into two new files:
  • external_libraries/ConfigureBaselibs.cmake: creates targets from a Baselibs install
  • external_libraries/ConfigureExternalLibraries.cmake: creates the same targets via find_package() for Spack or other non-Baselibs package managers
• FindBaselibs.cmake now only detects BASEDIR and sets Baselibs_FOUND
• esma.cmake automatically include()s the right one (ConfigureBaselibs or ConfigureExternalLibraries) based on Baselibs_FOUND, right after include(FindBaselibs)
• Updated CHANGELOG.md under [Unreleased]

This mirrors #559 (feature/v4-external-libs targeting release/v4), adapted for v5's single-precision FMS::fms target (no FMS::fms_r4/FMS::fms_r8 split, no FV_PRECISION-based component selection in find_package(FMS ...)).

Because the dispatch now lives in esma.cmake, downstream projects (like GEOSgcm) don't need to add anything to their top-level CMakeLists.txt. The only change needed there is to remove the old inline Spack/non-Baselibs block, e.g.:

find_package(MPI)

include(esma)

# Remove this whole block -- ConfigureBaselibs/ConfigureExternalLibraries
# is now included automatically by esma.cmake based on Baselibs_FOUND.
# if (NOT Baselibs_FOUND)
#   find_package(NetCDF REQUIRED C Fortran)
#   ...
# endif ()

ecbuild_declare_project()

Fixes since initial version

• Guard the historical ZLIB::zlib alias creation with if(NOT TARGET ZLIB::zlib) in ConfigureExternalLibraries.cmake; some ZLIB providers (e.g. zlib-ng's CMake config package) already export a target with that exact name.
• In ConfigureExternalLibraries.cmake, only look for FMS via find_package when FV_PRECISION is defined. Library projects like MAPL don't use FMS and don't set FV_PRECISION, so they're no longer forced to have it installed. This mirrors the existing FV_PRECISION convention already used in ConfigureBaselibs.cmake for the Baselibs case.
• In ConfigureBaselibs.cmake, enforce a minimum ESMF version (matching ConfigureExternalLibraries.cmake) by checking ESMF_VERSION explicitly, since Baselibs' esmf.mk has no CMake-style version string for find_package() to check.
• Added an ESMA_ESMF_MIN_VERSION variable (default 8.6.1), settable by a downstream project before include(esma), so projects with stricter ESMF requirements (e.g. MAPL3 needing 9.0.0) don't need their own version-guard boilerplate.

Test plan

• Being tested against a GEOSgcm branch with the old inline Spack block removed

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