Generate the initial migration for an entity from its model definition

[dob9601]

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.

[tyt2y3]

Just a point to note, create_table_from_entity returns a raw SQL when executed, but not a generic SeaQuery statement. So if you are targeting multiple databases, this won't work unless we embed three versions of the SQL, one for each backend.

[dob9601]

Maybe serialising the generated TableCreateStatement pre-execution could work - though I haven't looked at it's inner workings to see how viable it would be.

If it's something there's an interest in, I'm happy to try and whip up a PR for it