issue-37213: Standardizing FTPToS3Operator

[jroachgolf84]

Description

Per #37213:

There seems to be substantial discrepancies between transfer operators, especially those using S3 as a source or destination. For example, in the S3ToSFTPOperator, only a single file can be transferred from the SFTP to the S3 bucket. However, in the FTPToS3Operator an arbitrary number of files can be transferred from S3 to an FTP site. To add to this, the S3ToFTPOperator only supports a single file being transferred.

The PR refactors the FTPToS3Operator to mirror the functionality of the S3ToFTPOperator (it's "inverse" counterpart) and SFTPToS3Operator (it's sibling). This meant removing the ability to move multiple files and sticking with a single-file approach. This matches the behavior of its close relatives and helps to maintain Airflow DAG-authoring best practices.

Testing

These changes were tested E2E, as well as with updated unit tests. These unit tests can be run as follows:

git fetch origin
git checkout origin/issue-37213
git pull

breeze testing providers-tests providers/amazon/tests/unit/amazon/aws/transfers/test_ftp_to_s3.py

closes: #37213

[jroachgolf84]

@vincbeck, do you mind taking a peek at this one?

[vincbeck]

Those are breaking changes without any deprecation path. I do not think we can move forward with that. We usually first deprecate things, and then remove them

[jroachgolf84]

Would it be okay to go back and add deprecation for those parameters?

[vincbeck]

Would it be okay to go back and add deprecation for those parameters?

Definitely