Proposal: use {fmt} to convert Mantle types to string

As discussed in the sync on 2024-10-25, I would like to propose using the {fmt} library to convert Mantle types to string.

We would supply a seperate header with all specializations of fmt::formatter, and add a CMake option for formatting support. We would also add a dependent option for ostream support, using fmt::format to implement the ostream operators. This way, we would need to define the conversion to string only once.

This relates to !172, !165 (merged), and would also be an option to replace mantle_api::log_utils::ToStringView.

In the sync we found out, that most of the users already use {fmt} in their respective implementations, anyway. We still wanted to have a vote before adding another, albeit optional, dependency to Mantle.

Cheers, Martin