specify ad provider

[isstuev]

Description and Related Issue(s)

resolves https://github.com/blockscout/epics/issues/20

Proposed Changes

added NEXT_PUBLIC_AD_BANNER_ENABLE_SPECIFY variable to enable Specify ad provider for connected users

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request

[tom2drum]

What causes the changes of the banner margins on the screenshots? Is it correct that the banner is not aligned properly to the left or to the right side? Or is it an issue with the tests?

[isstuev]

What causes the changes of the banner margins on the screenshots? Is it correct that the banner is not aligned properly to the left or to the right side? Or is it an issue with the tests?

Since we're adding fallback logics here, we decided to unite ad formats for all providers to avoid layout shifts. Slise provides mobile format as 270x90 instead of 320x100 from all the other providers. We scale 270x90 to 300x100, but reserved slot for ad is 320x100. It adds the horizontal 'margins' that is approved by product.