Record Session start time

**Is your feature request related to a problem? Please describe.**
We need to uniquely identify sessions in the Admin Panel in a way that's meaningful for users.
Wallet name + session start date and time seems like a good way of doing so.

At the moment we can either derive the `captured_at` date/time for the earliest capture in the session (cumbersome), or use the `created_at` session time (potentially confusing for users).

**Describe the solution you'd like**
Record and upload the start time of each tracking session as part of the session information.

**Describe alternatives you've considered**
We could just derive a pseudo-start time using the first `captured_at` time in the bulk pack pipeline, but I'm not clear on whether the start of the session and first capture are considered as separate events in the app.

**Additional context**
Greenstand/treetracker-admin-client#896 describes the need to filter captures by session.
Eventually, we will add a _Tracking Sessions_ page in the Admin Panel that will display a summary of the session and act as a starting point for verification and analysis.

The issue names no files, tests, or entry points. Trace where tracking sessions are created and where session information is recorded and uploaded; done means each session includes its start time in the uploaded data for use in the Admin Panel.

Written by the indexing model from the issue text.