fix: verify Guest Authors when running CLI commands

[ronchambers]

Description

Co-authors-plus allows developers to use the filter add_filter( 'coauthors_guest_authors_enabled', '__return_false' ), but when developers implement this filter, many of the Co-Authors-Plus CLI commands will then throw FATAL PHP errors.

This PR will verify that when a developers uses the filter add_filter( 'coauthors_guest_authors_enabled', '__return_false' ), that the CLI commands that need Guest Authors will display an error and die gracefully as opposed to throwing a fatal error in the debug.log.

This PR may not be inclusive of all the Co-Authors-Plus CLI commands that need to verified. I simply searched for $coauthors_plus->guest_authors in each command; and if found, I added $this->verify_guest_authors_or_die();.

I did not fully test all commands nor all edge cases as I'm not a Co-Authors-Plus expert.

Example fatal error (without this PR) when running CLI commands:

PHP Fatal error:  Uncaught Error: Call to a member function get_guest_author_by() on null in wp-content/plugins/co-authors-plus/php/class-wp-cli.php:1087
Stack trace:
#0 wp-content/plugins/co-authors-plus/php/class-wp-cli.php(995): CoAuthorsPlus_Command->create_guest_author()
#1 [internal function]: CoAuthorsPlus_Command->create_author()

Deploy Notes

No new dependencies were added.

Steps to Test

1. Do not pull this PR yet.
2. Implement add_filter( 'coauthors_guest_authors_enabled', '__return_false' ) in a separate plugin.
3. Run Co-Authors-Plus CLI commands and see that they throw FATAL errors.
4. Pull this PR now.
5. Re-run CLI commands and see that FATALs are no longer thrown and instead WP_CLI will show an error message to the user (and die gracefully).

[leogermani]

This avoids the Fatals, but ideally these commands should work if guest authors are disabled.. :/ But that's I guess would require a bigger refactor