Add Datadog tracing interceptor contrib package

[eniko-dif]

This is a port of Datadog's internal Python tracing interceptor, used internally at Datadog with the Python SDK, adapted for open-source contribution. It has feature parity with the Go SDK's Datadog tracing interceptor, including deterministic span/trace IDs and replay-safe span emission, so traces stay coherent across worker restarts regardless of which SDK produced them.

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[tconley1428]

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