Add support for using S3-compatible blob storage providers in Bitbucket Server and GHES <3.8 migrations

When migrating from GitHub Enterprise Server <3.8 or Bitbucket Server/Data Center, the CLI uploads your migration archive to either AWS S3 or Azure Blob Storage.

Some GitHub customers use other blob storage services which are compatible with the AWS S3 API - common examples include [MinIO](https://min.io/) and [IBM Cloud Object Storage](https://www.ibm.com/cloud/object-storage).

We should add support for these providers. Broadly speaking, this _should_ just require an S3 API endpoint to be specified to override AWS's defaults, but we'd need to test that.