Add store information to OrganisationState

[StevenBlair123]

Support the following domain events:

StoreAddedEvent:

{
  "externalStoreCode": "55FFF0A8-6EDE-9A49-84-E0B33540B0BF9E",
  "externalStoreId": "BAT",
  "externalStoreNumber": "666",
  "organisationId": "066e0734-4ea2-480a-ac82-a5adb9e160fe",
  "storeId": "e4ced258-3ae6-8360-f014-1a57f5d44dae",
  "storeName": "SCO Bath"
}

StoreEnabledEvent:

{
  "organisationId": "066e0734-4ea2-480a-ac82-a5adb9e160fe",
  "storeId": "e4ced258-3ae6-8360-f014-1a57f5d44dae"
}

We should treat an event that already exists in state for a store as an update

When it comes to persistence we might want to review what it is we actually store, but for now, since the persistence is in-memory, we can look to at least hold the stores as a list / dictionary.