Source fetching should have a configurable exponential backoff.

When BuildStream fails to fetch a source, it tries again immediately. It would be a good feature for BuildStream to have an exponential backoff before proceeding to try again, to avoid rate-limiting.

No files or tests are named. Start by locating BuildStream’s source-fetch retry path and the existing configuration mechanism, then identify relevant tests for failed fetches. Done means the retry behavior uses configurable exponential backoff instead of retrying immediately, with tests covering the configured behavior.

Written by the indexing model from the issue text.