Import Tool: Add One-Shot Three Step Import

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.

Problem

Currently, the import tool does not do one-shot imports in the manner described in the design issue.

Solution

We should add a flag for the legacy import, essentially the (Import All) function. Preserving this prevents us from making massive changes to the .golden files we use to test this feature. Keeping these files untouched, at least for now, will allow us to detect real regressions brought on by this work with greater confidence.

Then we should then add a new exported method for the new one-shot import procedure, which will compose the work from Import Tool: Enable Full Manifest Pre Import (S... (#913 - closed) and Import Tool: Enable Blobs-Only Import (Step 3) (#912 - closed) and the existing imports into the procedure described by the design issue.