feat: add post author selection to Feed Imports

[HardeepAsrani]

Summary

This adds a select field to choose the author for the imported posts in Feed import. If the author is not available in the list, as we only load 100 users, a custom username can also be typed there.

Will affect visual aspect of the product

Yes

Screenshots

Test instructions

• We need to confirm the selected user is used as the author of imported posts.
• We should also make sure it works properly when upgrading from previous version.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/feedzy-rss-feeds-pro/issues/720.

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