Function to convert Go struct back to config.Value

[pietern]

Changes

This PR is the counterpart to #904. With this change, we are able to convert a config.Value into a Go struct, make modifications to the Go struct, and reflect those changes in a new config.Value.

This functionality allows us to incrementally introduce this configuration representation to existing bundle mutators. Bundle mutators expect a *bundle.Bundle argument and mutate its configuration directly. These mutations are not reflected in the corresponding config.Value (once introduced), which means we cannot use the config.Value as source of truth until we update all mutators. To address this, we can run convert.ToTyped and convert.FromTyped at the mutator boundary (from bundle.Apply) and capture changes made to the Go struct. Then we can incrementally make mutators aware of the config.Value configuration and have them mutate that structure directly.

Tests

New unit tests pass.

Manual spot checks against the bundle configuration type.

[mgyucht]

Hm, I'm curious. If we can convert a go struct into config.Value, what would be the difference between this and Merge(ref, ConvertToConfigValue(src))? It might actually be about the same?

[mgyucht]

Should we return ref here instead?

[pietern]

No; if the Go structure has nilled a pointer that wasn't nil before, it means that chunk of configuration (or pointer to primitive value) is no longer valid and should not show up in the returned config.Value.

[pietern]

Good point! It comes close but is not the same. Two differences come to mind:

1. Merge is additive and here we use zero-valued fields (primitive of nil pointers) to remove configuration.
2. By passing the original config.Value to convert.FromTyped we can retain location information for all values that were present in the original. Going from config.Value to Go structs and back is lossy and with this we can avoid loss for values that didn't change.