Make get_pings include locally defined services

[vladfedoriuk]

Summary

This pull request makes it possible to retrieve the pings from locally defined services, as discussed in https://github.com/hynek/svcs/discussions/81 and referenced to in https://github.com/hynek/svcs/issues/82.

Pull Request Check List

• [x] Typos aside (please, always submit typo fixes!), I understand that this pull request may be closed in case there was no previous discussion.
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• [x] Added tests for changed code.
  • The CI fails with less than 100% coverage.
• [x] New APIs are added to our typing tests at https://github.com/hynek/svcs/blob/main/tests/typing/.
• [ ] Updated documentation for changed code.
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