Add Scarf based telemetry

[kaxil]

To take the discussion about adding Telemetry forward, I am creating a draft PR that adds some basic telemetry to send to Scarf.

Voting thread: https://lists.apache.org/thread/h1x2glvnd42rbj2q2rgpfo3pjhmpt307

I have added docs on the data collection as well as a way to opt-out of it.

Telemetry added at:

• Scheduler startup Custom telemetry similar to other popular projects like Unstructured
• Webserver via a tracking pixel, similar to Apache Superset

Data collected:

• Airflow version
• Python version
• Platform System info: Linux/Darwin
• Machine type: arm64/aarch64
• Airflow Metadata DB: Postgres/MySQL
• DB: sqlite/postgres
• DB version: 12.6.3
• Number of DAGs

My proposal will be that this data will only be provided to PMC members on request to start with. For now, I am using the trial version.

We should also add it to the Airflow website similar to apache/superset#25639.

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