Use a better JSON serialization lib

[mesca]

Port metadata often contains "complex" objects (Datetimes, Numpy types, etc.) that are not easily serializable using the standard JSON module. The orjson library is a good candidate to tackle this issue, with the cost of introducing new dependencies. We should investigate further.

[bertrandlalo]

Two problems in this lib for our usage

• it only allows key of type string (but we sometimes use int) 'data': {1659675794020: {'A1': -0.01757812499999996, 'A2': -0.01757812499999996}....
• it does not convert pd.Timestamp, only datetimes