cephfs, tools/cephfs_mirror: migrate from boost::variant to std::variant

cephfs, tools/cephfs_mirror: migrate from boost::variant to std::variant

Replacing boost::variant with std::variant throughout the cephfs related
codebase. This change is part of our ongoing effort to reduce
third-party dependencies by leveraging C++ standard library
alternatives where possible.

Benefits include:
- Improved code readability and maintainability
- Reduced external dependency surface
- More consistent API usage with other components

Signed-off-by: Kefu Chai <tchaikov@gmail.com>