Dataset tags are currently only managed by processing handlers and can't be created "externally" (by the interface or an API)

We should add the possibility to add/remove them using an API.

-> Create a DatasetTagApi with a method addDatasetTag(Long datasetId, String tagName) in which we add a tag to a dataset.
(To see how it currently works, see OFSEPSeqIdHandler#setDatasetTags method -> Do not consider properties that are not relevant for this issue.)
-> Add a method removeDatasetTag(Long datasetId, String tagName) in which we remove this specific tag for this dataset.

The tag should be existing in the study (we currently base ourself on study tags, god knows why), and the used should have the right to import on the study / center.

Can also be done by Guewen :)