[PLAT-8732] Send queued sessions when connection regained

[nickdowell]

Goal

Make session delivery behaviour similar to event delivery.

Changeset

Schedules scan & upload of stored sessions upon regaining network connection.

Exposes the BSGSessionUploader object in order to call this from existing connectivity listener.

Testing

Not covered by automated tests. Verified manually using example app.

[github-actions]

Bugsnag.framework binary size did not change - 804,664 bytes

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  -0.3%     -82  -0.3%     -82    [__TEXT]
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