Add fallback browser header for when URL authentication fails

[emmambd]

Describe the problem

We're seeing an increase in URLs failing to run the validator (e.g #1813). #1914 is a good solution for technical users, but still won't be easy to use for non-technical audiences.

Proposed solution

As a smaller iteration on solving this problem, apply the method used in MobilityData/mobility-database-catalogs#702 to implement a fallback browser HTTP header in cases where our current header doesn't work.

Alternatives you've considered

No response

Additional context

No response