use :prefix to query attached databases

https://www.sqlite.org/lang_attach.html

the prefixed table name is intentionally not quoted because statements
expect a bare qualifier without quotes like `INSERT INTO
schema-name.table-name`

Author: sbaildon

lib/ecto/adapters/sqlite3/connection.ex

@@ -1946,7 +1946,7 @@ defmodule Ecto.Adapters.SQLite3.Connection do
   defp quote_table(nil, name), do: quote_entity(name)
 
   defp quote_table(prefix, _name) when is_atom(prefix) or is_binary(prefix) do
-    raise ArgumentError, "SQLite3 does not support table prefixes"
+    "#{prefix}.#{name}"
   end
 
   defp quote_table(_, name), do: quote_entity(name)