SFTPOperator - add support for list of file paths

[pauldalewilliams]

This PR adds support for passing a list of file paths to local_filepath or remote_filepath for the SFTPOperator. We've used this on my team for several years now to enable uploading or downloading several files via SFTP with a single connection in one task instead of splitting it out into multiple tasks.

It checks for and requires that the total number of paths provided is equal between the two parameters.
Maintains backward compatibility by converting a single string passed to the parameter into a list with that string as its only element.

[potiuk]

some tests are failing (Seems related)

[pauldalewilliams]

some tests are failing (Seems related)

@potiuk - This is related to our Slack conversation about the intermittently failing tests. If you look at the current checks you'll see it passed this time (after rebase). So it's not related to these code changes. I'm hoping to work on improving those 4 tests today if I can find the time.

[potiuk]

some tests are failing (Seems related)

@potiuk - This is related to our Slack conversation about the intermittently failing tests. If you look at the current checks you'll see it passed this time (after rebase). So it's not related to these code changes. I'm hoping to work on improving those 4 tests today if I can find the time.

Yeah. I recall. It was just strange they all failed there - they seemed related and I do not recall those tests to be THAT flaky :)

[pauldalewilliams]

some tests are failing (Seems related)

@potiuk - This is related to our Slack conversation about the intermittently failing tests. If you look at the current checks you'll see it passed this time (after rebase). So it's not related to these code changes. I'm hoping to work on improving those 4 tests today if I can find the time.

Yeah. I recall. It was just strange they all failed there - they seemed related and I do not recall those tests to be THAT flaky :)

What's funny is that they didn't fail in that run on the other variations (MySQL, MSSQL, Sqlite). That's what I'm talking about - sometimes you get lucky and they all pass, other times they don't. I'm going to rebase this again and we'll see if it passes, but I think my code in this PR is good to go...these tests just need to be tweaked.

[potiuk]

MySQL/MSSQL will not fail as they do not run provider's tests (those DBs take too much memory for Public Runners to run them quickly). But yeah - i was under the impression in our Slack conversation that they failed when they were run in isolation, but if they are flaky while running in CI, then indeed it needs to be fixed.