<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Adds a `confmap.Marshaler` implementation for `configoptional.Optional`
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Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Makes it so that unmarshaling into a `None[T]` is equivalent to
unmarshaling into a `(*T)(nil)`.
The goal is to be able to do #13109 without introducing any changes to
the behavior.
Needs #13161.
<!--Describe what testing was performed and which tests were added.-->
#### Testing
Add a unit test to compare the behavior of unmarshaling into `(*T)(nil)`
with that of unmarshaling into `None[T]`
<!--Describe the documentation added.-->
#### Documentation
Amend docstrings
<!--Please delete paragraphs that you did not use before submitting.-->
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Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>
<!--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