[TASK] Migrate AtlasMapping/DataSource and DocumentMetadata

[igarashitm]

For historical reasons the DocumentMetadata has been maintained separately from AtlasMapping mapping definition, and that's why AtlasMapping has DataSource inside it to carry a subset of properties from DocumentMetadata but keep the full DocumentMetadata isolated from the mapping definition.
However, after we introduced the ADM archive and allowed the data format modules to access DocumentMetadata directly, we no longer have a reason to keep DocumentMetadata and AtlasMapping mapping definition isolated. We should migrate them to avoid carrying duplicate properties, probably into AtlasMapping/DataSource and remove DocumentMetadata.

[stale]

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