Decouple support bundle inner layer from Nexus management

Replaces `BundleCollection.bundle: SupportBundle` with a slim
`BundleInfo { id, reason_for_creation }`. Moves the sled-storage
chunked transfer (`store_bundle_on_sled`), zip helpers
(`bundle_to_zipfile`, `recursively_add_directory_to_zipfile`,
`sha2_hash`), the `CHUNK_SIZE` and `TEMPDIR` constants, and the
DB-polling cancellation (`check_for_cancellation`) out of the inner
`support_bundle/` module and into `support_bundle_collector.rs`.

After this change the inner layer is a pure mechanism: it never reads
the `support_bundle` DB row, never talks to a sled-agent's bundle
storage endpoints, and treats CRDB only as a source of facts about
sleds, ereports, and blueprints. The outer collector remains the
manager of the bundle lifecycle.

This is the first step toward a future shared crate that omdb can use
to collect bundles when Nexus is down.

Author: smklein