SQL: issues with get_schema

[jorisvandenbossche]

• using get_schema in sqlalchemy mode (providing an engine) does also create the table, while just getting the create table string should not have any effects.
• the get_schema implementation for fallback duplicates a lot of code of PandasSQLTableLegacy._create_table_statement

[artemyk]

In addition, get_schema should return SQL with CREATE INDEX statements as appropriate. Need to add tests for this.

[artemyk]

@jorisvandenbossche Added an initial (and somewhat experimental) pull request to address this: #8232 . Thoughts?