Improved CustomBuildField for std::optional in IPC/libmultiprocess #251

The function now uses std::forward(value) when building the field, so rvalues are forwarded correctly instead of always being treated as lvalues. This resolves the existing FIXME in that code path.

Details Previously, the implementation forced every optional value to be accessed as an lvalue, which meant rvalues were unnecessarily copied. By applying perfect forwarding, the function now preserves the value category and allows moves where appropriate. The FIXME comment was removed since the intended behavior is now implemented

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Code review ACK 1977b9f3f658dc34dd9c7cd4eab38c0a9be6cc9a. Thanks for the fix! I'm guessing this change became possible after 3a96cdc18f2d1ca202fbc91551f27097fd7ec7f6 from #172 which started to support perfect forwarding