Introduce the View of MDL

[goldmedal]

Description

Introduce View

Now, we can support the view in the MDL.

  "views": [
    {
      "name": "customer_view",
      "statement": "select * from test.test.customer"
    }

User can store the Wren SQL as a view statement.

There're some limitations:

• The table name should be qualified ( full catalog.schema.table name )
• A complex expression should have alias name. There's an issue for the complex expression. The SQL as below can't be supported currently.

select order_id, sum(price) from wrenai.public.order_items group by order_id

Include SessionContext

In this PR, I include the Datafusion::SessionContext in the SQL planning flow. It will be responsible for store what we can use when planning SQL. I create another API named transform_sql_with_ctx that accepts a SessionContext which might be registered some tables. The original API transform_sql will create a default SessionContext for planning.

[grieve54706]

LGTM