Add PR title validation for Dex Auth operator repository using reusable workflow

[dariofaccin]

Context

Ensuring PR titles (and merge commits) always follow a defined naming convention is necessary to maintain a clean and readable repository.

Ref: canonical/bundle-kubeflow#1376

What needs to get done

1. Add new action, possibly named on_pull_request_lint_title, which uses the reusable workflow defined here

Definition of Done

1. Action runs when a PR is created, updated, reopened or synchronized
2. PR title validation is working

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-8226.

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