Rich Conditions - Use nlohmann::json

Based off of !3768 (merged) so diffs there will be seen here until that is merged.

Requires !3768 (merged), but does not have to be merged at the same time.

Det/LbDD4hep/include/LbDD4hep/ConditionAccessorHolder.h

@@ -21,6 +21,7 @@
 #include <GaudiKernel/detected.h>
 #include <Kernel/STLExtensions.h>
 #include <LbDD4hep/IDD4hepSvc.h>
+#include <LbDD4hep/utils.h>
 #include <any>
 #include <stdexcept>
 #include <type_traits>
@@ -139,7 +140,9 @@ namespace LHCb::Det::LbDD4hep {
           return T( cond );
         } else {
           using DT = std::decay_t<T>;
-          if constexpr ( !detail::PassAsAny<DT>::value ) {
+          if constexpr ( std::is_same_v<DT, nlohmann::json> ) {
+            return utils::y2j( cond.get<YAML::Node>() );
+          } else if constexpr ( !detail::PassAsAny<DT>::value ) {
             return cond.get<DT>();
           } else {
             const DT* p = nullptr;