support both naturalKey and referenceKeys declared through DomainObjects

[uladkasach]

it has become clear that for many valid domain objects we need to define more than one type of unique key:

• most often, the naturalKey is the only uniqueKey which needs to be declared (this is what is currently supported)
• however, often, one or more referenceKeys, which may be nullable and updatable, may also uniquely reference the domain object

we find the referenceKeys relevant most often when an external system also assigns a unique identifier to the entity which we want to track

• this referenceKey does not naturally identify the entity (since it is arbitrarily assigned by an external system at some point in time) (hence it is not a naturalKey of the entity)
• this referenceKey does uniquely identify the entity though (since each instance of a reference key is guaranteed to reference one and only one entity, when defined)

see below implementation and description

/**
 * the `DomainEntity.unique` property defines the keys upon which this entity is unique
 * - you must define the `natural key` of the domain entity
 * - you many optionally also define `reference keys` of the domain entity
 *
 * the `natural key`
 * - uniquely identifies the domain object based on its non-metadata properties
 * - this is required in order to identify distinct entities
 *   - this key must be defined for every entity
 *   - the values of this key must be present
 * - note
 *   - this may be a `uuid` if there are no keys upon which the entity is naturally unique on (unideal, but valid)
 *
 * the `reference key`
 * - when it exists, uniquely references a specific domain entity based on one its non-metadata properties
 * - this type of key
 *    - may not exist on a domain entity, it may not be relevant
 *    - may be nullable and updatable, starting with null until being instantiated at some point in the future  
 * - note
 *   - typically, these keys are properties known about the entity assigned by some external system to reference the entity
 *     - i.e.,
 *       - they uniquely reference the entity,
 *       - but the entity is not naturally identified by these properties
 *
 * for example
 * - a `GoogleAdsCampaign { account, name, googleReferenceName: string | null }` is likely unique `{ byNaturalKey: ['account', 'name'], byReferenceKeys: [['googleReferenceName']] }` since `account` and `name` are always present to identify it by, and `googleReferenceName` becomes a unique identifier for it once the campaign is created inside of the google-ads system
 * - an `InvaluableAuction { catalogRefId, catalogId, invoiceGroupId, name }` is likely unique `{ byNaturalKey: ['catalogRefId'], byReferenceKeys: [['catalogId'], ['invoiceGroupId' ]] }` since `catalogRefId` is always available, and `catalogId` and `invoiceGroupId` may or may not eventually be defined for it but are guaranteed to uniquely identify it once they are
 */
type unique = string[] | { byNaturalKey: string[], byReferenceKeys: string[] }

[uladkasach]

note unique: { onNaturalKey, onReferenceKeys } is better than { byNaturalKey, byReferenceKeys } since

findByUnique -> [findByUniqueOnNaturalKey, findByUniqueOnReferenceKey]

otherwise it'd be a double by and be more confusing