feat: Return "scraping attempts" from JS symbolication

[loewenheim]

This records the outcome of every attempt to scrape a JS source or sourcemap file from the web and returns these attempts in CompletedJsSymbolicationResponse.

It also records a "not attempted" attempt whenever a file is found via another method.

#skip-changelog

[github-actions]

	Warnings
⚠️	Snapshot changes likely affect Sentry tests. If the Sentry-Symbolicator Integration Tests in CI are failing for your PR, please check the symbolicator test suite in Sentry and update snapshots as needed.

Instructions for snapshot changes

Sentry runs a symbolicator integration test suite located at tests/symbolicator/. Changes in this PR will likely result in snapshot diffs in Sentry, which will break the master branch and in-progress PRs.

Follow these steps to update snapshots in Sentry:

1. Check out latest Sentry master and enable the virtualenv.
2. Enable symbolicator (symbolicator: true) in sentry via ~/.sentry/config.yml.
3. Make sure your other devservices are running via sentry devservices up --exclude symbolicator. If
   they're already running, stop symbolicator with sentry devservices down symbolicator. You want to use your
   own development symbolicator to update the snapshots.
4. Run your development symbolicator on port 3021, or whatever port symbolicator is configured to use
   in ~/.sentry/config.yml.
5. Export SENTRY_SNAPSHOTS_WRITEBACK=1 to automatically update the existing snapshots with your new
   results and run symbolicator tests with pytest (pytest tests/symbolicator).
6. Review snapshot changes locally, then create a PR to Sentry.
7. Merge the Symbolicator PR, then merge the Sentry PR.

Generated by 🚫 dangerJS against d75154c

[codecov]

Codecov Report

Merging #1311 (d75154c) into master (7ce9f63) will decrease coverage by 0.08%.
Report is 2 commits behind head on master.
The diff coverage is 65.47%.

@@            Coverage Diff             @@
##           master    #1311      +/-   ##
==========================================
- Coverage   75.75%   75.68%   -0.08%     
==========================================
  Files          91       91              
  Lines       14615    14691      +76     
==========================================
+ Hits        11072    11119      +47     
- Misses       3543     3572      +29