Support datetime type which preserve original input data

[fmassot]

Currently, when you declare a timestamp field, you can define several input formats and one output format. Internally, Quickwit parses the input value and stores the parsed value in the doc store.

On a search query, we thus return the datetime value in the defined output format.

We want to be able to return the same datetime value (same format) as the one in the original document that was indexed. (Note that this is the default behavior of Elasticsearch).