Add HTTPS Proxy Test (microsoft/vscode#235410)

.devcontainer/https-proxy-test/.gitignore

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+mitmproxy-config

.devcontainer/https-proxy-test/.vscode/tasks.json

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+{
+    "version": "2.0.0",
+    "tasks": [
+        {
+            "label": "mitmweb",
+            "type": "shell",
+            "command": "mitmweb --web-host '*'",
+            "runOptions": {
+                "runOn": "folderOpen"
+            }
+        }
+    ]
+}

.devcontainer/https-proxy-test/README.md

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+## HTTPS Proxy Test
+
+- `Dev Containers: Reopen in Container` > `HTTPS Proxy Test`.
+- First time: Install the certificate from `.devcontainer/https-proxy-test/mitmproxy-config` in the OS and restart VS Code. See https://docs.mitmproxy.org/stable/concepts-certificates/#installing-the-mitmproxy-ca-certificate-manually.
+- Add the user setting `"http.proxy": "https://localhost:8080"`.
+- Install GitHub Copilot Chat and use `Developer: GitHub Copilot Chat Diagnostics` to test connections with a HTTPS proxy. Use a second window to test connections from a local extension host.
+- Verify in the log terminal of the dev container that the proxy is being used.
+
+Note: Due to an issue in mitmproxy, Electron's `fetch` currently doesn't work. Add the user setting `"github.copilot.advanced.debug.useElectronFetcher": false` as a workaround.

.devcontainer/https-proxy-test/devcontainer.json

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+{
+	"name": "HTTPS Proxy Test",
+	"dockerComposeFile": "docker-compose.yml",
+	"service": "devcontainer",
+	"workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}/.devcontainer/https-proxy-test"
+}

.devcontainer/https-proxy-test/docker-compose.yml

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+version: '3.4'
+
+services:
+    devcontainer:
+        image: mitmproxy/mitmproxy
+        volumes:
+          - ../../..:/workspaces
+          - ./mitmproxy-config:/root/.mitmproxy
+        ports:
+            - "127.0.0.1:8080:8080"
+            - "127.0.0.1:8081:8081"
+        command: sleep inf

.gitignore

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-.vscode
 node_modules
 *.pem
 out