Remove spdlog from libmamba, provide libmamba-spdlog library

[Klaim]

Description

• removes spdlog from libmamba dependencies;
• adds a new library libmamba-spdlog which provides the spdlog-based LogHandler for mamba;
• libmambapy and mamba/micromamba now depends on libmamba-spdlog to keep using spdlog as a log handler;

For simplicity, libmamba-spdlog is a header-only library. This avoids many possible variations of libmamba usage to get in the way of easy packaging. As we always use spdlog as a header-only library and we just wrapps it, it shouldnt impact usage much. Also only end-user targets (usually executables) will need to decide which LogHandler to use so this header will probably only be compiled once.

Notes:

• this does not provide the necessary work to package libmamba-spdlog (next pr?);
• this is essentially solving what maint: Make spdlog a private dependency #3945 solves but relies on Logging impl separation #4016 and completely removes spdlog as a dependency of libmamba;
• this does not provide a way for libmambapy to use another logging implementation.

Type of Change

• Bugfix
• Feature / enhancement
• CI / Documentation
• Maintenance

Checklist

• My code follows the general style and conventions of the codebase, ensuring consistency
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run pre-commit run --all locally in the source folder and confirmed that there are no linter errors.
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes