Add BulkImports API functions and tests

What does this MR do?

Adds support for the Bulk Imports API by implementing the following functions:

• ListBulkImports — GET /bulk_imports
• ListBulkImportEntities — GET /bulk_imports/entities
• GetBulkImport — GET /bulk_imports/:id
• ListBulkImportEntitiesByID — GET /bulk_imports/:id/entities
• GetBulkImportEntity — GET /bulk_imports/:id/entities/:entity_id
• GetBulkImportEntityFailures — GET /bulk_imports/:id/entities/:entity_id/failures
• CancelBulkImport — POST /bulk_imports/:id/cancel

Testing

Added tests for all new functions in bulk_imports_test.go.

Related issues

#2080 (closed)