ENH/SQL: Allow 'dtype' argument in to_sql() to take dictionary including a default datatype
Related to #11886

Could we modify the dictionary interpretation in `to_sql()` so that the datatype specified by a key like `'default'` would apply to all other columns not already specified in the dictionary? Or, alternatively, we could break this out to two different arguments: a dictionary for specific columns and a datatype for all the rest?

Instead of choosing between specifying every column having a specific type and using one datatype for all columns, I need a hybrid of the two.