Add interface to load metadata

[swhume]

Add an API/interface for providing the metadata needed to address the metadata gaps identified in Phase 1, as well as other study-specific metadata.

[swhume]

We decided to implement placeholders in the generated define.xml files that will be replaced with the missing metadata values. This is how we address the metadata gaps found in Phase 1. We can now move on to a new feature that replaces placeholders with study-level metadata.