Add Slug for Campaign URL

[versun]

Is your feature request related to a problem? Please describe.
Currently, when publishing a campaign, the URL address is automatically generated using a UUID.
However, this may not be user-friendly or memorable for customers who want to visit the campaign page.

Describe the solution you'd like
I would like to request the ability to add a custom slug to campaigns before publishing.
This would allow users to create their own URL addresses that are more user-friendly and memorable.
For example, instead of a URL like "https://demo.listmonk.app/archive/468bbc7f-6880-4d61-bde0-5f6728ebb790", users could create a URL like "https://demo.listmonk.app/archive/welcome-to-listmonk". This would make it easier for customers to remember and share the URL with others.

Overall, I believe this feature would improve the user experience for both campaign creators and customers, as it would allow for more personalized and memorable URLs.

[knadh]

Whew, this was a non-trivial addition: 0d319ad