Complete NGSI-LD backward and forward support

[chicco785]

Is your feature request related to a problem? Please describe. PR #373 allows not to inject NGSI-LD using the existing NGSIv2 endpoint.

Still the following limitations apply:

• [x] temporal attributes are not supported (#395) - what is relevant here is that this attributes are used to create the time index of the series
• [ ] other attributes may be added as well in future (not a priority probably, may not be tackled) (#396 )
• [ ] context is currently not stored (#468 ).
• [ ] datasetIds are not supported (#552 ).
• [ ] query endpoints returns NGSIv2 data types.

Describe the solution you'd like

• temporal attributes should be supported
• context should be stored
• a specific endpoint for NGSI-LD should be created, including support for NGSI-LD temporal queries, despite some of the semantic there seems odd to us (e.g. only number values are tracked in timeseries, thus changes on other attributes over time are not captured)

[c0c0n3]

Notes

• This issue supplants #154. Close that issue too when merging the PR implementing the current issue.
• For additional context look at the discussion about NGSI-LD on #373, e.g. this thread.
• NGSI-LD does not really support a temporal model w/r/t entity graphs, so to claim QL implements the NGSI-LD entity graph model is enough to store entities the way we do at the moment---notice PR #373 added the Relationship type. See this comment for more details about it.

[c0c0n3]

Also, we've got to add modifiedAt and observedAt to the list of attrs we use to compute the time index, see #433.

[github-actions[bot]]

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