Allow WebDriver BiDi to override the cache mode

The primary use case here is allowing WebDriver to enforce that requests do not
use the network cache. But for future extension this works by allowing WebDriver
to override the cache mode on requests.

fetch.bs

@@ -5616,6 +5616,8 @@ run these steps:
    <a>default `<code>User-Agent</code>` value</a>) to <var>httpRequest</var>'s
    <a for=request>header list</a>.
 
+  <li><p>Run the <a>WebDriver BiDi update request cache mode</a> steps with <var>request</var>.
+
    <li><p>If <var>httpRequest</var>'s <a for=request>cache mode</a> is "<code>default</code>" and
    <var>httpRequest</var>'s <a for=request>header list</a> <a for="header list">contains</a>
    `<code>If-Modified-Since</code>`,