rust(feat): Adds a new SiftStream mode for only backup file writes

[tsift]

In order to support a SiftStreamMode that only writes data to backup files, a bit of refactoring is also included in this PR. The set of changes includes:

• Moving code shared between the IngestionConfigMode and FileBackupMode implementations to common locations (either moving internal state up to SiftStream itself, or simply shared modules).
• Moving some file writing implementations to shared locations for use between FileBackupMode and AsyncBackupManager.
• The SiftStreamMode trait was expanded for the various send APIs and was "sealed". The API that will be interacted with by users of the crate will be those on SiftStream itself now, while the "mode" implementations are internal details (though technically public due to the generic type in SiftStream).

Documentation was updated, including a new example which uses the new SiftStreamMode (I also added example for using the FlowBuilder into both).

Finally, I also went through and updated some pub items to pub(crate).

[nathan-sift]

Nice, the new <Encoder, Transport> looks good, and should hopefully be easier to extend in the future.