Support custom "reverse requests" from debug adapters

Debug adapters can send events that are not part of the DAP spec that can be handled by extensions by subscribing to [`vs.debug.onDidReceiveDebugSessionCustomEvent`](https://code.visualstudio.com/api/references/vscode-api#debug.onDidReceiveDebugSessionCustomEvent). But events are not the only thing that DAs can send, they can also send ["reverse requests"](https://microsoft.github.io/debug-adapter-protocol/specification#reverse-requests) such as `runInTerminalRequest`. It's not currently possible to create custom reverse requests because there is no hook like `onDidReceiveDebugSessionCustomEvent` for handling custom requests. This can be useful when a debug adapter needs to get some information from the extension - for example if a debug adapter is launching a web app and needs to expose a URI through `vscode.env.asExternalUri()` and get back the public URI. Right now this has to be done by firing an event from the DA and then having the extension send back another event (which feels awkward given reverse requests already exist in DAP). @weinand I filed this here because I think it's a VS Code API that's missing and not something for the DAP spec, but if it makes more sense somewhere else like (https://github.com/Microsoft/debug-adapter-protocol), let me know and I can re-raise it. Thanks!