Feature Request: AutoMigrate supports placement of geo-partitioning

Now the AutoMigrate does not support the placement in `Create Table` SQL statement.

Please refer to the following SQL statement

```
CREATE TABLE Companies (
id INT64 NOT NULL,
create_at timestamp,
update_at timestamp,
name STRING(MAX) NOT NULL,
Location STRING(MAX) NOT NULL PLACEMENT KEY
) PRIMARY KEY(id)
```

Please add the support to placement key in the AutoMigrate tool.

Start by locating the AutoMigrate implementation that generates CREATE TABLE SQL and trace how column definitions are rendered. Check how the placement key is represented in the provided statement, then add coverage for a table using PLACEMENT KEY and verify that AutoMigrate emits valid SQL supporting geo-partitioning.

Written by the indexing model from the issue text.