Fix #8556: Add GPC toggle

[cuba]

Summary of Changes

This pull request fixes #8556

Submitter Checklist:

• [ ] Unit Tests are updated to cover new or changed functionality
• [x] User-facing strings use NSLocalizableString()
• [x] New or updated UI has been tested across:
  • [x] Light & dark mode
  • [x] Different size classes (iPhone, landscape, iPad)
  • [x] Different dynamic type sizes

Test Plan:

Test using console (navigator.globalPrivacyControl should return the appropriate value)

Note: There is no support for the header value yet

Screenshots:

Reviewer Checklist:

• [ ] Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• [ ] Necessary security reviews have taken place.
• [ ] Adequate unit test coverage exists to prevent regressions.
• [ ] Adequate test plan exists for QA to validate (if applicable).
• [ ] Issue and pull request is assigned to a milestone (should happen at merge time).

[ShivanKaul]

@cuba can you add screenshots?

[cuba]

@cuba can you add screenshots?

Uploaded

[diracdeltas]

cc @Brave-Matt could you update our privacy user support document to mention this? something like On iOS, GPC is only enabled via the JavaScript API for now, which can be disabled in Settings under Brave Shields and Privacy.