JCLOUDS-457: BlobStore MultiPartUpload strategy

The code related to the MultiPartUpload strategy has been added.
MultiPart uploads use an upload strategy (e.g. sequential vs parallel)
and also a slicing strategy to split the payload in different parts.
In my humble opinion, this may be appropriate if we have in mind that we could make a multipart upload strategy to be able to re-upload some parts if we get an error.

Maybe I could pass the ratio as an argument so the user can have some control for this?