[nlohmann-json] Add port feature to control implicit conversions behaviour

[thomasgt]

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Is your feature request related to a problem? Please describe.
The nlohmann json library is gradually backtracking on providing implicit conversions to their types via operator T(). They've added a CMake option (JSON_ImplicitConversions) to control this behaviour. I think we should expose this option as a feature on the nlohmann-json port.

For context, here are some relevant issues and PRs in the nlohmann json project:

• operator T() considered harmful nlohmann/json#958
• Feat/explicit conversion operator nlohmann/json#1559

Proposed solution
Add a new port version that exposes this option. The library currently defaults to enabling implicit conversions, but I wouldn't be surprised if they change the default in the future. I think we should ensure the default behaviour in vcpkg matches that of the library, so perhaps it's best to frame it as an inverted feature (e.g. "disable-implicit-conversions") and leave it out of the port's default features. When the library changes their default from JSON_ImplicitConversions=ON to JSON_ImplicitConversions=OFF, we should add disable-implicit-conversions to the default features for the port.

Describe alternatives you've considered
Alternatively, we could make a non-inverted feature (e.g. implicit-conversions or enable-implicit-conversions), include it in the default features now, and then remove it from the default features when/if the library changes its defaults.