Reference data from multiple tenants in "activerecord-multitenant"

hi! I use this gem all the time, thanks!
By the way, I have a suggestion, could you please incorporate multi-tenant reference support into this gem?
The reason is that there is a case in which we need to acquire data across tenants, but we are having trouble achieving this with “activerecord-multitenant” at present.
To give a concrete example, we have a case where a group company is using an application and wants to be able to check data across multiple tenants in the group company.

The sample code is as follows.

```ruby
tenant1, tenant2, tenant3 = Tenant.order(:created_at).take(3)
pp tenant1.id # => "1310041a-c8a8-4d35-b395-8386af95a24f"
pp tenant2.id # => "bb257a59-9811-473b-87a7-64c11f8799ab"
pp tenant3.id # => "eff47159-53af-4b35-ba39-13e6fac0d5fa"

# Multiple `tenant1.id, tenant2.id, tenant3.id` queries are added
MultiTenant.with_read_multiple([tenant1.id, tenant2.id, tenant3.id]) do
puts User.all.to_sql
# => SELECT "users".*
# FROM "users"
# WHERE "users"."tenant_id" IN ('1310041a-c8a8-4d35-b395-8386af95a24f', 'bb257a59-9811-473b-87a7-64c11f8799ab', 'eff47159-53af-4b35-ba39-13e6fac0d5fa')
end
```

At this point, reading alone is sufficient, so we are not considering support up to writing

I'll make a PR if there is interest or acceptance of this proposal!