[Core] TraitsData::getTraitProperty should not be [[nodiscard]]

[feltech]

What

Remove [[nodiscard]] attribute from TraitsData::getTraitProperty, or otherwise redesign getTraitProperty.

Why

The design of getTraitProperty uses an out-parameter for the value the user is most interested in. The return value is a bool indicating whether the property is set. A possible valid use case is ignoring the return value, e.g. in a "value or default" flow:

openassetio::Str value = "fallback";
traitsData->getTraitProperty(&value, traitId, key);

where value will be the stored value if set, or will remain "fallback" if not. In this case, the returned bool is not interesting and can be safely ignored.