Allow returns from all single client proxies

[halter73]

We should delete SingleClientProxy<THub> in favor of SingleClientProxyWithInvoke<THub> for simplicity if nothing else. We should also switch to T Single(string connectionId) => Client(connectionId); because that now works in our case. We wouldn't even have to implement it.

I see the inconsistent experience with strongly-typed vs. non-strongly-typed hubs as a complete non-issue. It's FAR better to be "inconsistent" here! We're already being inconsistent by returning the exact same type from both Single() and Client() in the strongly-typed case instead of two different types (namely ISingleClientProxy and IClientProxy) in the default case.

I understand that we can't change IHubClients : IHubClients<IClientProxy> to IHubClients : IHubClients<ISingleClientProxy> in the default case without breaking apps. It's unfortunate, but if we could do that, we wouldn't have had to introduced Single() in the first place.

However, that doesn't mean we should nerf strongly-typed hubs! So what if they are more flexible with their Client property? Single() and Client() return the exact same type for strong typed hubs. It's already different.

Follow up to #41763 (comment). Hopefully the change and the tests make it clear what I'm trying to fix.

[MackinnonBuck]

/azp run

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