Add ads status header to all search.brave.com requests.

[aseren]

Previously, X-Brave-Ads-Enabled: 1 header is added to search.brave.com main frame navigation requests if Brave Ads are enabled. It was implemented here: brave/brave-browser#25430
Now server search side requires to get X-Brave-Ads-Enabled: 1 header with xhr/fetch requests also. And for simplicity, it was decided to send the header with all search.brave.com requests if Brave Ads are enabled.

The X-Brave-Ads-Enabled header description: https://github.com/brave/brave-browser/wiki/Custom-Headers#x-brave-ads-enabled

Resolves brave/brave-browser#26812

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
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• 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

Test Plan:

Run Brave browser with fresh profile

• Open https://search.brave.com
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com
• type query in the search box and press submit
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com and that a search result ad can be viewed
• do a search query from Omnibox
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com and that a search result ad can be viewed

Enable Brave Private Ads

• Open https://search.brave.com
• make sure that X-Brave-Ads-Enabled: 1 header is presented in requests to search.brave.com
• type query in the search box and press submit
• make sure that X-Brave-Ads-Enabled: 1 header is presented in requests to search.brave.com and that a search result ad is not shown
• do a search query from Omnibox
• make sure that X-Brave-Ads-Enabled: 1 header is presented in requests to search.brave.com and that a search result ad is not shown

Disable Brave Private Ads

• Open https://search.brave.com
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com
• type query in the search box and press submit
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com and that a search result ad can be viewed
• do a search query from Omnibox
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com and that a search result ad can be viewed

Run browser in Private mode

• Open https://search.brave.com
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com
• type query in the search box and press submit
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com and that a search result ad can be viewed
• do a search query from Omnibox
• make sure that X-Brave-Ads-Enabled header is not set in requests to search.brave.com and that a search result ad can be viewed

Also need to make sure that this logic is working for dev/staging Brave Search environments. The list of supported hosts is here: hosts

[tmancey]

LGTM