Move items preparation code to a service.

Normalizers are used in serialization which I don't think it applies here. But this could be another case where we could use tagged services to handle items preparation based on the type of field.

The current code however definitely does not belong to the api service where it currently is.

Setting back to needs work.

Branch rebased and I have moved the code to a converter service that transforms item objects to documents. Furthermore we do not need to deal with converting values to a proper format as search api module already provides the `DataType` plugins which are responsible for outputting the values in a proper type and format.

Ready for review.

Tests are passing, so see no issue why this can be merged. This also then affects the numeric search/sort 📌 Sorting and sort boost Fixed as the switch that converts the values was removed, but should work (as the integer conversion was removed for that reason), so marking it as RTBC.

Rebased, however there is a problem, a test fails because the fulltext fields are not correctly indexed as the value returned from the field is not string but a TextValue object and that gets indexed as an empty JSON. It does look that after all we will need to implement tagged services to handle such cases.

I have added a tagged services mechanism for converting special types such as TextDataType to whatever format is acceptable in Meilisearch.

Ready for testing

Branch was rebased, which means should have all the latest changes, including the sorting by integer which does work (tested also manually), so tests are passing and see no issue why this can be merged, RTBC.

Merged, thanks everyone.

Automatically closed - issue fixed for 2 weeks with no activity.