Redshift alter column type no set

[yoavcloud]

Expanded the support for ALTER TABLE tbl ALTER COLUMN col TYPE <type> to the Redshift dialect in addition to PostgreSQL. Generalized this option using the supports_xxx functions in the Dialect trait to maintain backwards compatibility with expected error messages in non-supporting dialects.

Improved SQL output from Ast by introducing an option to determine if SET DATA appears before the TYPE keyword.

[iffyio]

Suggested change

	/// Returns true if the dialect supports `ALTER TABLE tbl ALTER COLUMN col SET DATA TYPE <type> USING <exp>`
	/// Returns true if the dialect supports the `USING` clause in an `ALTER COLUMN` statement.
	/// Example:
	/// ```sql
	/// ALTER TABLE tbl ALTER COLUMN col SET DATA TYPE <type> USING <exp>`
	/// ```

thinking something like this so that its a bit clearer which part of the statement the flag refers to?

[iffyio]

not sure I followed that this flag is necessary, it looks like we would be able to accept either variant with or without the SET DATA prefix? The error message of existing tests if I understood the PR description correctly we can change to match the new behavior I think

[yoavcloud]

Yes that is an option. I hesitated a bit because saw specific negative tests, and did not want to break anything.

[iffyio]

Suggested change

	/// Whether the statement included the optional `SET DATA` keywords
	had_set: bool,
	/// Set to true if the statement includes the `SET DATA TYPE` keywords
	set_data_type: bool,