refactor: have only one descriptors object

Right now, the generated code has a non-trivial descriptor object such as

```ts
class Client {
...
descriptors: {
page: { ... },
stream: { ... },
longrunning: { ... },
batching: { ... },
}
...
}
```

This is actually an overcomplicated object since a method can only be paging, or streaming, or longrunning, or support batching, but not two of those at the same time. We need to replace it with just one descriptors object (key: method name, value: any descriptor).

This will formally be a breaking change because someone could depend on a given descriptor present (we actually gave some advice about that long time ago), so this should wait till the next major version bump.