Add new filter to show or not ElasticPress Indexing option

[Rahmon]

Description of the Change

This change adds a new filter to show or not the ElasticPress Indexing option in the Sites screen (wp-admin/network/sites.php).

Alternate Designs

Benefits

Possible Drawbacks

Verification Process

This has been tested manually. Adding the code below is expected that the ElasitcPress Indexing option doesn't appear in the Sites screen.

add_filter( 'ep_show_indexing_option_on_multisite', '__return_false' );

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Closes: #1893

Changelog Entry

Added the filter ep_show_indexing_option_on_multisite.