Uniquely identifying services via an external identifier

[mrshll1001]

Continued from each https://github.com/openreferral/api-specification/issues/35 and https://github.com/openreferral/api-specification/issues/77

Noting that we have external identifiers for org-ids and locations already, so this follows precedent. It will likely need a discussion of what registers of services look like.

[MikeThacker1]

This conversation is relevant to matching records from multiple (federated) sources.

V3 has the organization_identifier entity - see here in the international schema reference and here in the data model for the UK profile.

In the UK I'd expect the identifier_scheme to reference registers such as those maintained by Companies House and by the Charity Commission.