Add support for running a "direct" migration (without blob storage) from GHES

In the GraphQL API, it is possible to run a "direct" migration from GitHub AE or GHES to GitHub Enterprise Cloud. In so-called "direct" mode, GHEC connects to the migration source directly to perform the data export, rather than the CLI performing the expert and then uploading the data to a blob storage provider ("indirect" migration).

Right now, this is not supported in the CLI. If you specify the `--ghes-api-url` argument to `gh gei migrate-repo` in order to point to your target instance, then the presence of this argument will cause the CLI to use the "indirect" blob storage flow.

We should add a way for a user to specifically request a direct migration when specifying an API URL. This might also be a good time to rename this attribute to be more generic, rather than GHES-specific. When making this change, we should be careful of backwards compatibiltiy.