Enhance SQLAchemy backend insert implementation to include usage of table_expr for inserting data

[harsharaj96]

Apart from inserting data using a dataframe or by giving a table name, if a user gives table expression as input then insert operation must be performed into the desired table.

[datapythonista]

Not for the current work, but for the future, I think it probably makes more sense to have a single argument source that can be a dataframe, an expression, another table.... and perform the operation based on the type. Instead of having a different argument for each source kind.

[harsharaj96]

Using the obj variable in the .insert() function to take either pandas DataFrame or table_name or ibis TableExpr like what we wanted to do with the source variable. Added test cases for this scenario as well.