Preserve MoA field from source OpenTargets

**Is your feature request related to a problem? Please describe.**
Currently the MoA gets classified into `activates` or `inhibits` leading to information loss, for instance:
- is this direct or indirect activity regulation?
- is the compound an antagonist or inverse agonist?
https://github.com/BioDataFuse/pyBiodatafuse/blob/5fb176515ca4bae988cd3c17d6f485977d908543/src/pyBiodatafuse/annotators/opentargets.py#L532-L535

**Describe the solution you'd like**
Preserve the field to allow for more precise mappings. The current binary classification can be kept, it is useful. Some [existing properties from `RO`](http://purl.obolibrary.org/obo/RO_0002448) could be used to model the precise MoAs in the rdf module.