Inconsistent Multipart Nomenclature

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The object_store crate inconsistently used multipart multi-part and multi part to refer to the ability to upload in chunks.

Describe the solution you'd like

We should mirror the AWS naming, and using multipart as a single word - https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateMultipartUpload.html

Describe alternatives you've considered

Additional context

[tustvold]

label_issue.py automatically added labels {'object-store'} from #5500