Update and add test for diagnostic settings

[jaredfholgate]

Description

This updates the diagnostic settings code and adds a test / example for it.

Closes #75

Type of Change

• [ ] Non-module change (e.g. CI/CD, documentation, etc.)
• [x] Azure Verified Module updates:
  • [x] Bugfix containing backwards compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in locals.version.tf.json:
  • [x] Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
  • [ ] The bug was found by the module author, and no one has opened an issue to report it yet.
  • [ ] Feature update backwards compatible feature updates, and I have bumped the MINOR version in locals.version.tf.json.
  • [ ] Breaking changes and I have bumped the MAJOR version in locals.version.tf.json.
  • [ ] Update to documentation

Checklist

• [x] I'm sure there are no other open Pull Requests for the same update/change
• [x] My corresponding pipelines / checks run clean and green without any errors or warnings
• [x] I did run all pre-commit checks

[microsoft-github-policy-service[bot]]

[!IMPORTANT] The "Needs: Triage :mag:" label must be removed once the triage process is complete!

[!NOTE] This label was added as per ITA06.