Fix to merge code from `_create_table_statement` and `get_schema`

Reorganized a lot of the SQL schema creation code.  Now database table is not dropped / create when `PandasSQLTable` object is instantiated, but instead when `insert` method is called on it.  This allow us to `get_schema` to generate `CREATE TABLE` code without actually creating the table.
(Includes updates from #8083, so will have to be rebased when that is merged)
Is this needed? (I should look at it again, but certainly possible that I forgot to add this one when I added schema support). But if so, it would be a bug now, and so a test should be added that catches that (and preferably done in e seperate commit)