feat: Allow per-service Datadog APM enable for edxapp

[timmc-edx]

This will allow us to enable Datadog for edxapp independently of other services during the migration. It also pushes the knowledge of the two common switches into the defaults layer so that only one variable needs to be consulted. Similar work for other services would follow.

I've also switched to a topic-first rather than an ENABLE-first naming convention since it tends to sort and group better.

This supports edx/edx-arch-experiments#632

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.