Ensure config structs can be marshaled into YAML

[evan-bradley]

Is your feature request related to a problem? Please describe.

It is possible for structs to contain fields that cannot be marshaled into YAML. We recently went through the process of updating or removing some of these (e.g. #10310), but there are currently no tests in place to ensure non-marshalable fields will be added in the future.

In particular, I want do so something like this:

// Programmatically-generated config
cfg := otelcol.Config{...}

// Should marshal to YAML
conf := confmap.New()
conf.Marshal(cfg)

// Should be equivalent to cfg
cfg2 := unmarshal(conf)

Describe the solution you'd like

Add tests to mdatagen that test that we can marshal/unmarshal all config structs. Ideally, we put the config structs through an iteration of marshaling and unmarshaling to ensure the structs can't easily enter a state where conversion is impossible.

Describe alternatives you've considered

Some kind of integration test that happens in a secondary package could be a possible way to do this, and would forgo the need for a direct confmap dependency in every package that for use only in tests.