ci: only trigger e2e test on pr comment

[yanksyoon]

Applicable spec:

Overview

• Only trigger expensive e2e test on Pull Request comment containing: "ci: run e2e". PRs and issues are same types in GitHub hence the issue_comment trigger.
• The required status check for Required E2E Test Status Checks already exists on a repository level, hence this can be safely implemented.

Rationale

• The e2e tests are costly and are resource intensive. When a PR is ready, we can trigger the test run via a PR comment.

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.
• The application version number is updated in github-runner-manager/pyproject.toml.

No code changes, ci changes only.