Fix: Make CopyTo logical plan output schema consistent with physical schema

[bert-beyondloops]

Which issue does this PR close?

• Closes Output schema of the CopyTo logical plan is not correct. #16704

Rationale for this change

Schema of logical plan should be the same as the associated physical plan.

What changes are included in this PR?

1. The logical plan schema() method for a CopyTo plan has been changed.
2. Construction of the CopyTo structure has been replaced with a "new" method and all callers have been updated.
3. A new test has been written to verify equality of the schemas of the CopyTo logical plan with the associated physical plan. See test_copy_schema on dataframe level. e0e88a1#diff-4a599584dfc900ec21169f4f820a1b1db46b004b77533dab83a6178d5d3a467eR6143

Are these changes tested?

A new test has been written to verify that both the logical plan as the physical plan return the same schema.

Are there any user-facing changes?

On SQL level, there is no change ; you cannot construct a sql statement where a CopyTo logical plan is child from another plan.

On dataframe level, retrieving the schema of a CopyTo logical plan will provide a correct schema now.

[alamb]

Makes sense to me -- thank you @bert-beyondloops

[bert-beyondloops]

unused import removed.

[alamb]

Thanks again @bert-beyondloops