Add bulk imports service with start migration method

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This MR introduces support to start migrations using bulk imports (also known as direct transfer).

This is my first contribution to the project. I'm happy to make adjustments.

I'm not quite sure what the exact status of this API is. While the banner says "Migrating projects with this API is in beta", migrating projects is only one feature. The other feature, migrating groups, appears to be stable.

Under "Start a new group or project migration", it says:

• entities[group_entity] to migrate a group.
• entities[project_entity] to migrate a project. (Status: Beta)

I don't mind if you decide not to merge this until the whole API is stable.

Relates to #2080.