cli: add acquire_cli (#179837)

* cli: add acquire_cli

As given in my draft document, pipes a CLI of the given platform to the
specified process, for example:

```js
const cmd = await rpc.call('acquire_cli', {
	command: 'node',
	args: [
		'-e',
		'process.stdin.pipe(fs.createWriteStream("c:/users/conno/downloads/hello-cli"))',
	],
	platform: Platform.LinuxX64,
	quality: 'insider',
});
```

It genericizes caching so that the CLI is also cached on the host, just
like servers.

* fix bug

cli/src/lib.rs

@@ -18,6 +18,7 @@ pub mod tunnels;
 pub mod update_service;
 pub mod util;
 
+mod download_cache;
 mod async_pipe;
 mod json_rpc;
 mod msgpack_rpc;