<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Adds `omitempty` to all struct fields that are not set with any of the
`NewDefault[...]` functions. If we like the approach here, I'll open PRs
for other config modules.
<!-- Issue number if applicable -->
#### Link to tracking issue
Works toward
https://github.com/open-telemetry/opentelemetry-collector/issues/12191
<!--Describe what testing was performed and which tests were added.-->
#### Testing
I added tests to check that this works when structs are marshaled with
confmap in a separate module so as not to create a direct dependency on
confmap within confignet.
Evan Bradley