Create Unique ID for Blobs

What does this MR do and why?

Currently a GraphQL query to list blobs from the given tree returns blob ids which are not unique. In #356006 (closed) it was discussed that a combination of blob web_path and sha would be a suitable replacement for ID.

This MR:

• Creates a unique ID for blobs by using the Blob attributes 'web_path/sha' as id.
• Moves the Object ID (formally the id) to its own property oid.
• Adds oid to a spec checking for an expected list of graphql fields.

Contributes to #356006 (closed)

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