Add "RSA_NETWORK_MODE" constant for Network Mode

[sanketio]

Description of the Change

Add new RSA_NETWORK_MODE constant to define default setting for network mode for multisite.

Closes #317

How to test the Change

1. Network activate the plugin on multisite
2. Add define( 'RSA_NETWORK_MODE', 'enforce' ); or define( 'RSA_NETWORK_MODE', 'default' ); to your site's wp-config.php file.
3. It should use that setting across the network.

Changelog Entry

Added - New RSA_NETWORK_MODE constant to define default setting for network mode for multisite

Credits

Props @sanketio

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.