feat(internal): span sampling file config envar (backport #4042)

[mergify[bot]]

This is an automatic backport of pull request #4042 done by Mergify. Cherry-pick of c0e9a9bc3a355c41634d8958a41893b33050d683 has failed:

On branch mergify/bp/1.4/pr-4042
Your branch is up to date with 'origin/1.4'.

You are currently cherry-picking commit c0e9a9bc.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ddtrace/constants.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   ddtrace/internal/sampling.py
	deleted by us:   tests/tracer/test_single_span_sampling_rules.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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