Add optional confidence score on TranscriptionEntity

Please add a nullable float confidence field on arkindex.documents.models.TranscriptionEntity.

This field should be exposed on those endpoints:

• ListTranscriptionEntities (in output)
• CreateTranscriptionEntity (in input)

Don't forget to add some test cases in unit tests for both missing field, null value, float value, and invalid value (which should give a 400).

We need to support missing field to avoid workers breakage