Adopt Bedrock Client abstractions in SignalR client

[analogrelay]

Epic #10869

SignalR currently uses the "Bedrock" abstractions on the server side (ConnectionContext, etc.) but has a custom client-side abstraction (IConnectionFactory). Bedrock has evolved to support a client-side abstraction now so SignalR should update to use it (since it would be a breaking change).

The new client abstraction shape is:

public interface IConnectionFactory
{
    ValueTask<ConnectionContext> ConnectAsync(System.Net.EndPoint endpoint, CancellationToken cancellationToken = default);
}

Some elements to this:

• Add IConnectionFactory (as above) somewhere (Connections.Abstractions? or a new library?)
• Add HttpEndpont sub-class of System.Net.Endpoint (for use in the IConnectionFactory.ConnectAsync method).
• Configure the TransferFormat in the constructor instead of in the ConnectAsync method?
• Delete existing connection factory and change to use the new abstraction.

[analogrelay]

TBH, this is another thing making me nervous :).

cc @davidfowl to review my assessment of the elements needed to finish this.

[davidfowl]

This looks good. We need to rename the interface to something that says client.

[analogrelay]