Improve rest upload of explorer distributions

Working on automation (NXP-29371), it appears that current rest api is not easy to use to upload a distribution in "one shot" while setting up the distribution name, version, aliases, etc...: only the export content is taken into account for now.

Note when uploading in the UI, there is a "validation" step, error-prone, as even before validation, the distribution is visible to users. Reserved distribution names are not checked at this moment, and not all available properties can be edited in the API -> factor this out with the update API.

Also check the "override" mechanism when saving or uploading a distribution: saving with the same name overrides, while uploading does not, and it should be made consistent (not overriding: better use the delete api to do so).