Import Tool: Enable Blobs-Only Import (Step 3)

Context

As part of Design an In Place Migration Procedure for Self... (#884 - closed) we described a three-step import for self-managed instances moving to database metadata. As part of this, danging blobs will be imported last after a full import of all other registry data.

Problem

Currently, the import tool does not allow only blobs to be imported.

Solution

We should create a flag for that only imports blobs. We can later roll this into the more high-level behavior described in the design issue.

In Import All we can check for this flag and complete the import here In order to skip to the end cleanly, we'll probably need to extract the blob import logic as well as the following repository import logic after this point into their own functions. This also sets us up for future work aligning the tool with what's described in the design issue.