v3.4.0: feat: Add datadog_diagnostics plugin app (#722)

See https://github.com/edx/edx-arch-experiments/issues/692

Testing setup:
https://2u-internal.atlassian.net/wiki/spaces/ENG/pages/1173618788/Running+Datadog+in+devstack

And then in lms-shell:

```
make requirements
pip install ddtrace
pip install -e /edx/src/archexp/
./wrap-datadog.sh ./server.sh
```

Expect to see this log message:
`Attached MissingSpanProccessor for Datadog diagnostics`

NOTE: This prints "Spans created = 0; spans finished = 0" in devstack when
shut down with ctrl-c, but not when restarted due to autoreload (where it
prints correct info). Something is initializing Django twice and one span
processor is getting span info while the other is printing at shutdown.
There's more to debug here, but it seems stable enough to least try
deploying it.