feat(app): add link between traces + rum

[seaerchin]

Problem

currently our backend trace isn't tied to our frontend RUM replay. this PR adds an allow list of backend urls, which will then allow DD to tie the traces to the specific rum session.

Closes [insert issue #]

Solution

• add allowedTracingUrls - see here
  • we can add in regex, functions or strings.
  • strings match by initial section, so any url that starts with the given string will be matched. in our case, we use the env vrs REACT_APP_BACKEND_URL