Initial work on LogEvent support

[solnic]

This is initial work to support logs in envelopes.

TODO

• Add LogEvent item type
• Add support for log events to Transport
• Add Sentry.capture_log
• Add support for Sentry.logger enhanced with capturing logs

Refs #2600

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[solnic]

This would grow too fast too big, so I'm closing it in favor of smaller PRs. Starting with #2606