New firebase test lab run Mixpanel event

[zuziaka]

Create new Mixpanel event: firebase test lab run.

Properties:

• schema_version
• flank_version
• project_id
• app_id
• device_type
• cost.virtual
• cost.physical
• cost.total
• shard_count
• tests.total
• tests.successful
• tests.failed
• tests.flaky
• test_duration.total
• test_duration.physical
• test_duration.virtual
• outcome - outcome of the entire run (see: https://github.com/Flank/flank/blob/3a213579b0b8ed7ca018314f0619146408292b35/docs/feature/summary_output.md#possible-outputs)
• configuration names/values (see: https://github.com/Flank/flank/blob/master/docs/flank_mixpanel_metrics.md#configuration)

Also:

• Create documentation with schema versions
• Each property change should be documented and associated with schema version bump