Add `mapping_set_id` to JSON response

[anitacaron]

Do it the same way as in #34.

Make sure that the other field is already in JSON format.

Need to change this in the mapping_set:

{
      "object_source": "http://purl.obolibrary.org/obo/hp",
      "uuid": "0025bf7bc8d551c4b4f2c34d6e89d58f",
      "subject_source": "http://purl.obolibrary.org/obo/mp",
      "other": "PMID:32585259",
      "mapping_date": "2022-02-01",
      "comment": "KidsFirst; see ticket https://github.com/mgijax/mammalian-phenotype-ontology/issues/3497",
      ...
      "mappings": {
        "href": "http://localhost:8008/mapping_sets/0025bf7bc8d551c4b4f2c34d6e89d58f/mappings"
      }
    }

[anitacaron]

A possible solution, in case the field other has something:

• If the content in other is JSON, add the key sssom_api:mapping_set_id
• If not, add the content in other to the key other_sssom, and add the key sssom_api:mapping_set_id

[anitacaron]

After some meetings, it was decided not to change the SSSOM model. The solution is to have endpoints only for the UI; we can add any field important for the UI and one that is SSSOM conform.