Use vscode_extension_marketplace settings in Web IDE

Issue: Update Web IDE settings to use admin settings f... (#508991 - closed)

• Previous MR: Stub vscode extension marketplace UI (!181624 - merged)

What does this MR do and why?

This MR integrates the application setting vscode_extension_marketplace into the Web IDE and User Preferences page:

1. It adds to the Web IDE settings pipeline:
   • extension_marketplace_generator.rb which generates the setting for vscode_extension_marketplace. Previously this was hardcoded with OpenVSX defaults, but now we read from application settings (based on vscode_extension_marketplace_settings feature flag).
   • extension_marketplace_home_url_generator.rb which reads from vscode_extension_marketplace setting and returns the "Home URL" (used in the user-opt in text). This is moved to the settings pipeline, because we'll need to use this value in the extension_marketplace_metadata_generator.rb (future MR) to determine the user opt-in.
2. It updates the WebIde::ExtensionMarketplace.marketplace_home_url function to use the settings pipeline to pull vscode_extension_marketplace_home_url.
3. It updates other parts of the code to respect the vscode_extension_marketplace_settings feature flag which means that we are not hardcoding Open VSX marketplace, and instead use the value from the vscode_extension_marketplace application setting.

Screenshots or screen recordings

Here's a demo where I walk through the behavior:

web_ide_extension_marketplace_demo

How to set up and validate locally

1. Enable vscode_web_ide, web_ide_extensions_marketplace, and vscode_extension_marketplace_settings feature flags.
2. Go to Admin > General > VS Code Extension Marketplace.
3. In the textarea (UI handled in separate MR) enter {}.
4. EXPECTATION: In the user preferences, the checkbox for extension marketplace should not be visible. (You might need to refresh the page a couple of times since things get heavily cached in GitLab).
5. EXPECTATION: In the Web IDE, open the extension marketplace tab. The extension marketplace should be disabled.

6. ---

7. Go to Admin > General > VS Code Extension Marketplace.
8. In the textarea enter {"enabled":true,"preset":"open_vsx"}.
9. EXPECTATION: In the user preferences, the checkbox for extension marketplace should be visible, with links pointing to https://open-vsx.org.
10. EXPECTATION: In the Web IDE, open the extension marketplace tab. The extension marketplace should be enabled and reaching out to https://open-vsx.org.

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How to setup local Open VSX proxy for more testing

This isn't required to test out this MR, but for the brave:

1. Make sure nginx is installed.

2. Update /etc/hosts with openvsx.test 127.0.0.1

3. Create a directory and create a file openvsx-proxy.conf with content:

   events {
     worker_connections  1024;
   }
   
   http {
     server {
       listen openvsx.test:8883 ssl;
       server_name openvsx.test;
    
       ssl_certificate openvsx.test.pem;
       ssl_certificate_key openvsx.test-key.pem;
   
       location / {
         proxy_pass https://open-vsx.org/;
       }
     }
   }
   

4. Inside the directory, run mkcert openvsx.test.

5. Start an nginx server with something like nginx -c ./openvsx-proxy.conf

6. Try visiting https://nginx.test:8883 locally to confirm if the server is up and running.

7. Now you can use the following custom_values in your vscode_extension_marketplace settings on the GDK:

   {
     "enabled":true,
     "preset":"custom",
     "custom_values": {
       "service_url":"https://openvsx.test:8883/vscode/gallery",
       "item_url":"https://openvsx.test:8883/vscode/item",
       "resource_url_template":"https://openvsx.test:8883/vscode/unpkg/{publisher}/{name}/{version}/{path}"
     }
   }