Add Identifiers table

[MikeThacker1]

OpenCommunity discovery work recommended adding a a new data structure for identifiers such as

• Companies House reference
• Charity Number

This approach is good for finding duplicates across multiple ‘data custodians’.

iStandUK says that this is similar to the structure originally proposed by LGA in version 1 of their Locally Delivered Services schema (teh CSV schema). This approach allows for codes to be used that are not URIs. The LGA / Open Referral UK spec should move to this.

We've not changed the initial Open Referral UK spec to add this as it came up late in 2019/20 work, but I think it should be a high priority for the next version.

There are no backwards compatibility issues.