Translate System.Text.Json JSONPATH APIs when they become available

[roji]

dotnet/runtime#31068 tracks adding JSONPATH querying APIs in System.Text.Json. If and when these become available, we could translate them. This would be useful for weakly-typed documents where the schema isn't stable/known in advance.

Note #2570 which is about LINQ to JSONPATH (for strongly-typed access with stable schemas) and #2668 which provides raw JSONPATH string querying.