DataProviderSwitchAccountMapping

One pre-filled old->new pair (or a lone account we could not pair), with its confidence.

source_account is null for a leftover new account; target_account is null for an old account with no match (kept as manual).

type DataProviderSwitchAccountMapping {
  confidence: DataProviderSwitchMappingConfidence!
  sourceAccount: Account
  targetAccount: Account
}