Include the source from which the configuration was loaded as part of the config metadata

If the same configuration is repeated across multiple sources, it can be confusing to determine where the config originated from.

This is especially true for those who are not familiar with the priority order of the config loaders, making it even more confusing.

To prevent such issues, let's add metadata to each config and make sure the config loader loads this metadata along with the config itself.

For example, if a certain setting exists in both `manager.toml` and `etcd`, and the "from" field of that setting indicates `etcd`, it would clearly show that the setting was ultimately overridden by the value from `etcd`.

JIRA Issue: BA-1537