ENH: Option to not add 'mastDownload' to download_path in Observations.download_products

It's great that since #1150 it's possible to download data to a custom directory. However, `'mastDownload'` is always appended to the download path even if it isn't the current directory.

It would be nice if there was an option to not do this to keep an existing directory structure cleaner. I was thinking about implementing this myself but it's hard to ensure a good default backwards compatible behavior since the default would change depending the value of the `flat` flag.

Inspect Observations.download_products and its handling of flat and custom paths first. Confirm the intended option and backward-compatible defaults for both flat values; done means callers can keep or omit the mastDownload directory without an unintended change to existing behavior.

Written by the indexing model from the issue text.