Export metadata keys that the get fn can work with

[jleach]

When fetching the legacy did the get fn below needs a key. Its not exported by AFJ so it needs to be looked up and passed in. Would be handy to export it as an enum or similar.

      const legacyDidKey = '_internal/legacyDid'

      const didRecord = await didRepository.getById(record.connectionRecord!.did!)
      const did = didRecord.metadata.get(legacyDidKey)!.unqualifiedDid