Generate the initial migration for an entity from its model definition

Motivation

Creating an initial migration for a model can be quite tedious, especially if the model is complex. It would save a lot of time if the initial migration to create the table could be generated from the model definition.

Proposed Solutions

Expand on the functionality of sea-orm-cli. Arguably create_table_from_entity already does the majority of the legwork in terms of creating the TableCreateStatement. The only thing that would need doing is dumping the generated code into a migrations file.

Current Workarounds

1. Convert the generated TableCreateStatement into a SQL string for the chosen backend.
2. Convert the SQL string into a StatementBuilder (can be done by playing around with the StatementBuilder trait and Statement::from_sql_and_values.
3. Use exec_stmt() to run this SQL.

Start by reading sea-orm-cli and the Schema::create_table_from_entity API linked in the issue to understand how the TableCreateStatement is produced. The feature is done when the CLI can generate migration-file code from an entity model definition, with the generated table creation statement represented in the migration rather than only executed as SQL.