feat: Implement structlog batch exports logger

[tomasfarias]

Problem

The existing logging implementation for batch exports is a bit clunky:

• We hardcode the log source to "batch_exports", but it would be nice to distinguish from backfills.
• Logs are not rendered as JSON, so they would not be parse-able by log aggregators (Loki).
• Logs repeat themselves: Every log line says the export type.
• All the code is in the generic batch_exports.py module.
• Kafka logging handler is threaded instead of async. Having an async handler would allow us to not block other tasks (mostly heartbeats).
• We rely on global variables (yuck).

Changes

• Move over to structlog to handle logging (structlog is already a dependency from the django-structlog requirement, so nothing new here, but I did bump the version, which should be supported). This allows us to set context variables (like team_id and destination) once and for all.
• Moreover, structlog allows us to render logs as JSON, so log aggregators will be happy.
• Move logger to its own module.
• Use aiokafka to make kafka producer async.

Overall, the logging system now looks easier to understand and, eventually, extend. The line diff may be deceptive as a lot of it is in docstrings to aid with understanding and more than half is in unit testing (which we didn't properly have much of).

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How did you test this code?

Added unit tests.

[tomasfarias]

Going to keep this one in draft until we merge #18380 and we can rebase on that before addressing test failures.

[tomasfarias]

Closed in favor of: #18458