fix: validate urls when creating a notification endpoint

[jeffreyssmith2nd]

• Closes NotificationEndpoint: http type does not check if URL string follows URL schema  #23724

Description

Validates that a proper http url is being used. This should help prevent notifications from (semi)-silently failing in the background.

Context

The value add is to help users make sure their notification endpoints actually work, so they don't miss important notifications.

Note for reviewers:

Check the semantic commit type:

• Feat: a feature with user-visible changes
• Fix: a bug fix that we might tell a user “upgrade to get this fix for your issue”
• Chore: version bumps, internal doc (e.g. README) changes, code comment updates, code formatting fixes… must not be user facing (except dependency version changes)
• Build: build script changes, CI config changes, build tool updates
• Refactor: non-user-visible refactoring
• Check the PR title: we should be able to put this as a one-liner in the release notes