config: value tracing

[bt-justtrack]

To determine where a config value is coming from ( especially useful when looking at a service's config http page) we want to store a "trace" where that setting is coming from (eg ENV, config.dist.yml, ...).

[bt-justtrack]

Plan: add a property

type ValueSource string
const (
ZeroValueSource ValueSource = "zero value"
DefaultValueSource ValueSource = "default value"
AdditionalDefaultValueSource ValueSource = "additional default value"
)
...
source ValueSource

And set that on load to where that is coming from, eg config.dist.yml or env + env var
For defaults we want to:
UnmarshalWithDefaultsFromKey => source should be the source config key
UnmarshalWithDefaultForKey => source should be the code file location of the passed settings struct

source gets overwritten on merge according to provider priority.