Missing permission check for Omnisend plugin configuration page

[atiq-bs23]

The Omnisend plugin configuration page does not verify whether the current user has the MANAGE_PLUGINS permission.
As a result, users without this permission can still access the configuration page directly via URL.

This behavior may lead to unauthorized access to plugin configuration settings.

Steps to Reproduce:

1. Create a new role.
2. Assign Admin area access to the role.
3. Ensure the role does NOT have the MANAGE_PLUGINS permission.
4. Log in with a user assigned to this role.
5. Manually navigate to: https://{YOUR_DOMAIN}/Admin/Omnisend/Configure

Actual Result
The Omnisend configuration page is displayed successfully.

Expected Result
Access should be denied.

Suggested Fix
Add a permission check for MANAGE_PLUGINS in the Omnisend plugin configuration controller/action.