<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
<!-- Issue number if applicable -->
Adds new `configoptional` module.
I left `GetOrInsertDefault` out of this first PR so we can get agreement
on the basics first.
#### Link to tracking issue
Fixes#12981Fixes#10266
<!--Describe what testing was performed and which tests were added.-->
#### Testing
<!--Describe the documentation added.-->
See #13018 for usage and testing of the package on `confighttp` and
`otlpreceiver`.
Pablo Baeyens