Change auth header to be x-kagi-authorization

Instead of using the standard Authorization header, we instead want to
use a custom one.
This is because the standard authorization header, when sent to some of
our subdomains (like help.kagi.com), actually break since its backed by
a GCS bucket which reads that header.

This should remove any conflicts, and we already support this header on
our side :)

shared/src/background.js

@@ -129,7 +129,7 @@ async function updateRules() {
           type: 'modifyHeaders',
           requestHeaders: [
             {
-              header: 'Authorization',
+              header: 'X-Kagi-Authorization',
               value: sessionToken,
               operation: 'set',
             },