chore: add an option to block network requests to server (sourcegraph#8151)

Add a new feature flag to allow disabling network requests from Cody, in
the rare cases that Cody client causes problems. Both vscode and agent
uses the patched `DelegateAgent`, so this flag should apply to both.

Cody does not use `keepAlive`, nor `keepAliveMsecs` (which defaults to
1s), so this flag should be reflected ~1 hour or so after set on server
side.

## Test plan

-- manual testing with s2

Author: fengzhou-sg

lib/shared/src/experimentation/FeatureFlagProvider.ts

@@ -162,6 +162,11 @@ export enum FeatureFlag {
      * When set to true, context will not be added to chat automatically.
      */
     AgenticContextDisabled = 'agentic-context-disabled',
+
+    /**
+     * Disable network request to servers, in case of bad client
+     */
+    DisableNetworkRequests = 'disable-network-requests',
 }
 
 const ONE_HOUR = 60 * 60 * 1000