Add webhook format check for Slack

[Hailong-am]

Add Slack webhook URL format check here
https://github.com/opensearch-project/notifications/blob/main/notifications/notifications/src/main/kotlin/org/opensearch/notifications/index/ConfigIndexingActions.kt#L59

it should have same url format check as Microsoft teams

[bblazek98]

Hello, could i be assigned to this issue?

[zhichao-aws]

If we change this validation, we need to modify UT/IT in this repo, as well as in dashboards-notifications. The mock data doesn't have the correct format, and this will fail dashboards-notifications cypress test

[Noir01]

Can I be assigned to this, seeing how bblazek98 doesn't seem to be interested?

[SuZhou-Joe]

Can I be assigned to this, seeing how bblazek98 doesn't seem to be interested?

Sure, assigned.