Connectors: Support custom plugin is_active callback in connector registry and plugin status output

[iamadisingh]

This changeset backports connector improvements from Gutenberg to Core:

• Extend connector plugin metadata to support optional plugin.is_active callback.
• Validate plugin.is_active as callable during WP_Connector_Registry::register().
• Use plugin.is_active callback when building connector script-module data, with fallback to file_exists + is_plugin_active.
• Update default Akismet connector to use a robust activity check: defined( AKISMET_VERSION ) && class_exists( Akismet, false ).

Some plugins (especially MU-loaded or non-standard load paths) cannot be reliably detected via is_plugin_active(file) alone. Allowing an explicit activity callback provides accurate activation state in connectors UI.

[gaisma22]

No patch file or PR is attached to this ticket. The description contains code snippets but nothing that can be applied or tested. Removing has-patch and needs-testing. Adding dev-feedback as the changes need review from a developer with commit access first.

Backport of wordpress/gutenberg#76994. Adds an optional is_active callable to the plugin definition when
registering a connector via WP_Connector_Registry::register().

The Connectors screen previously resolved a connector's active/installed status by checking is_plugin_active() and file_exists() against WP_PLUGIN_DIR only. Plugins installed as must-use plugins — or loaded from non-standard paths could not be detected this way, causing theirconnectors to disappear from the screen even though the plugin was in use.

Trac ticket: https://core.trac.wordpress.org/ticket/65020

This PR has been updated to also address https://core.trac.wordpress.org/ticket/65099.