speedreader: Add --speedreader-backend switch

[kkuehlz]

Resolves brave/brave-browser#14959

This is so @pes10k and @rebron can test the changes we make on nightly. Not particularly relevant until #8349 is merged because the current heuristics backend in master is very incomplete.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

[pes10k]

Gotcha, thanks for clarifying. Could you change the names to something more descriptive then?

Maybe list, classify (and if there is such an option, list-and-classify)?

[kkuehlz]

Done. Since we only have two backends right now, any option that isn't classify is implied to be list, including no cli option at all.

[pes10k]

great, thanks! LGTM!

[kkuehlz]

iOS failing for python reasons, CI passes on macOS, didn't touch anything in vendor/brave-ios. Checked with ios folks and said it should be good to ignore. pre-init failing because of python linting, so also unrelated.