Allow multiple identifiers for objects in spec

[lambda-crime]

Currently the specification limits the amount of identifiers to one, which doesn't represent the reality of e.g. train stations which sometimes have dozens upon dozens of identifiers assigned by many different sources.

Thus I would suggest adding the field additionalIds to the spec as an optional field containing a list of Identifier Objects that every object type can have.

Identifier Objects would look similar to the way Träwelling represents identifiers:

{
  type: "identifier",
  identifierType: "IBNR", // The type of identifier (e.g. IBNR, RIL100, &c)
  identifier: "8000105",
  operator: ... // optional, operator id or operator object - identifies the operator that assigned the identifier
}