feat: include text widgets in nrql search

[rgraber]

Description:

Updates (and renames) new_relic_nrql_search.py to search in markdown widgets as well as NRQL queries. Left as unreleased because it doesn't actually affect use of the plugin.

GH Issue:

https://github.com/edx/edx-arch-experiments/issues/323

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ ] Version bumped
• [x] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

Author concerns:

List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.