[Feat]: Allow turning rewrites off or give better rewrites control over asExternalUri

## What is your suggestion?
Allow controlling how `asExternalUri()` rewrites work beyond current `--proxy-domain` capabilities or allow turning them off completely.

## Why do you want this feature?
Extensions like Dart Code use `asExternalUri` to open a WebView in the editor with their DevTools. In Dart Code's case, it refuses to function unless it's served off `http://localhost:port` root, and also requires connection to its WebSocket server on another port.

I use manual SSH port forwarding for these ports and it used to work about a year ago, but now code-server rewrites these URLs with its own port forwarding, which doesn't work with the extension and I found no way to fix it.

## Are there any workarounds to get this functionality today?
- I tried configuring `.example.com` port forwarding, but it doesn't work with the extension
- I can't set `--proxy-domain http://localhost:{{port}}` because it results in `https://httplocalhost%3Aport` URLs
- With `--proxy-domain localhost:{{port}}` VS Code fails to connect to workbench (The workbench failed to connect to the server (Error: Failed to construct 'URL': Invalid URL))
- Setting `CS_DISABLE_PROXY=1 && code-server` and deleting all the existing port forwards doesn't turn the rewrites off
- Setting `remote.forwardOnOpen` and `remote.autoForwardPorts` to `false` doesn't turn the rewrites off.
- Maybe I'm doing something wrong, but setting `VSCODE_PROXY_URI=http://localhost:{{port}}` environment variable doesn't produce any result

With `CS_DISABLE_PROXY=1` environment variable, `remote.forwardOnOpen` and `remote.autoForwardPorts` set to `false`, when I set a breakpoint on [mainThreadWindow.ts:65](https://github.com/microsoft/vscode/blob/main/src/vs/workbench/api/browser/mainThreadWindow.ts#L65), I get the following structure:
```js
{
authority: "example.com",
fragment: "",
path: "/proxy/9100/",
query: "",
scheme: "https",
}
```

Some implementation details not very related to the problem, but useful to know: When debugging, I get the aforementioned structure twice, first for the DevTools URL, and the similar one for the debug service URL. While the debug service URL is actually a websocket connection, Dart Code uses http protocol to get the `asExternalUri()` and then replaces it with ws, since the method doesn't support ws port forwarding.

After the extension receives the mapped url, a webview opens at `https://example.com/DEBUGGER_URL`, dropping the `/proxy/9100` part, which I saw mentioned in the docs, though I don't understand how it works. The webview contains only `Not Found` error.

When using subdomain port forwarding, devtools just fail to load because of some bug in them.

When I copy the URL to my browser and manually change `https://example.com` to `http://localhost:` (in both URI itself and its URI parameter that contains the endpoint for the debug service), the DevTools load and successfully connect to the websocket port I SSH forwarded earlier.

When opening VS Code with port forwarding over `http://localhost:8080`, I get the following structures from `asExternalUri`:
```js
{
"external": "http://127.0.0.1:9100/DEBUGGER_URL",
"path": "/network",
"scheme": "http",
"authority": "127.0.0.1:9100",
}
```

Which is exactly what I need. But the resulting webview still points to `localhost:8080`, so the URL gets patched on some stage, but I can't find any lines in Dart Code which would cause that. Unless I'm missing something, my suspicion is that code-server replaces the result somewhere on returning the result to the extension or displaying the webview, but I don't know how to debug the remote side of the extension and need help with that.

The extension also has "Launch DevTools in browser" option, which tries to open `https://example.com/proxy//` when opening VS Code from `example.com`, and `http://localhost:8080/proxy//` when opened from localhost. If I studied the extension's code correctly, it uses [openExternal()](https://github.com/Dart-Code/Dart-Code/blob/22043d8838bfdc2106e05ad6501e8fcad8de9e08/src/shared/vscode/utils.ts#L234) method.

Thus my suggested solution is to either allow to specify any sort of origin template in `--proxy-domain`, like `http://localhost:{{port}}`, or at least allow rewrites to be turned off so the localhost URIs remain untouched.

## Are you interested in submitting a PR for this?
I could try, but I'm not familiar with code-server's code base yet.