[SignalR] Strongly-typed Hub Proxies

Edits from @anurse

Let's use this to track strongly-typed hub proxies all-up. We've talked about a few ways of doing it (code-gen, reflection emit, etc.). We have to consider a few things

* Which clients *can* support it?
* Which client environments can support it (for example, AOT environments like Unity wouldn't be able to support reflection emit-based generation)?
* What's the API
* How does this interact with client-side hubs #5278

Original issue follows
---

On the server side, we can create a strongly-typed hub (docs: https://docs.microsoft.com/en-us/aspnet/core/signalr/hubs?view=aspnetcore-3.0#strongly-typed-hubs). However, on the client-side, everything is still loosely typed, which means we lose type safety. Does the TypeScript client have any support for a strongly-typed connection? I imagine it could use generics on the `build` function, eg:
```
const connection = new HubConnectionBuilder()
.withUrl("/hub")
.build();
```

My ideal setup would be automatically generating a TypeScript client based on the C# strongly-typed Hub, but for now I'd be fine just manually creating a TypeScript type that replicates the server-side one.