Sightings uses `uint` to reference `string` identifiers

[0xThiebaut]

While Attribute and Event's id attribute are represented by a string (misp-core-format#L304 and misp-core-format#L94), Sighting mentions they should be referred-to as unsigned integers. Is it intended to have an uint reference a string?

Per misp-core-format#L1074-L1084:

id, event_id and attribute_id MAY be present.

id represents the human-readable identifier of the sighting reference which belongs to a specific MISP instance.
event_id represents the human-readable identifier of the event referenced by the sighting and belongs to a specific MISP instance.
attribute_id represents the human-readable identifier of the attribute referenced by the sighting and belongs to a specific MISP instance.

org_id MAY be present along the JSON object describing the organisation. If the org_id is not present, the sighting is considered as anonymised.

org_id represents the human-readable identifier of the organisation which did the sighting and belongs to a specific MISP instance.

A human-readable identifier MUST be represented as an unsigned integer.