Rename `exportable` variables to `portable` for `BulkImports`

What does this MR do?

This MR:

• Is part of #329864 (closed) effort
• Renames BulkImports::Exports to BulkImports::FileTransfer instead, in order for the configs to be used later on during import process, which is not related to export (part of work to be done in #329864 (closed))
• Renames exportable variables to portable, which captures both exportable and importable, since some classes are used during both import and export. Renaming exportable to portable unifies terminology across export & import (which is currently not the case for traditional Project/Group Import/Export)

Mentions #329864 (closed)

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