Add initial http via lookup support

[mrchrisadams]

This pull request introduces a new feature to handle 'via' HTTP headers in the FileFinder class and updates the corresponding tests. The changes include adding a method to check for 'via' headers, modifying the URI resolution logic to use this method, and adding new test cases to verify the functionality.

New Feature: Handling 'via' HTTP Headers

• src/carbon_txt/finders.py: Added a new method _check_for_via_delegation to check for 'via' headers in the HTTP response and return the URL specified in the header if present.
• src/carbon_txt/finders.py: Updated the resolve_uri method to use _check_for_via_delegation for handling 'via' headers and to raise an exception if the HTTP request fails.

Test Updates

• tests/test_finders.py: Modified existing test cases to use variables for domain and URL strings for better readability.
• tests/test_finders.py: Added a new test case test_looking_up_uri_with_delegation_using_via to verify that the resolve_uri method correctly follows the URL specified in the 'via' header.