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Ex. Adding a feature - Explain what this achieves.-->
#### Description
<!-- Issue number if applicable -->
Adds `GetOrInsertDefault` method to `configoptional.Optional`. Uses it
in tests.
We originally did not do this because we only had use cases for this in
tests. However #13856 shows that this is useful in programmatic use
cases as well.
#### Link to tracking issue
Fixes#13856
d1a5e5abb8