use datafusion plan_to_sql function instead of expr_to_sql

[hozan23]

Hello,

Following on #18, #99 and #102, we are thinking to use the plan_to_sql function within the SqlTable.scan() implementation, instead of generating the SQL query using the sql() function from SqlExec.

The proposed approach is:

• From the scan() function -> create a new logical plan -> pass it to the plan_to_sql function -> pass the generated SQL string to SqlExec.

One concern with this approach is that we may lose the ability to implement custom logic for each engine. A possible solution would be to use the CustomDialect for engines that don't have supported dialects in DataFusion.

cc: @phillipleblanc, @backkem

[phillipleblanc]

This makes sense to me.

[hozan23]

Resolved with #106