feat: Add EDXAPP_DDTRACE_PIP_SPEC to specify ddtrace version for edxapp

[timmc-edx]

This allows specifying the entire requirement spec, not just the version, because we want to be able install from HTTPS URLs or other sources, not just PyPI. (This will allow us to install a release candidate.)

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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.