Avoid allocations in ClientWebSocket.ConnectAsync in the common case

[MihaZupan]

ClientWebSocketOptions.Proxy is set by default, so the shared handler wasn't being used in the common case.
This PR adds a second handler field that's used when a default proxy is requested.
While we're changing this code, I also changed it to store an HttpMessageInvoker instead of SocketsHttpHandler.

For the following test case

usingvarcws=newClientWebSocket();awaitcws.ConnectAsync(uri,CancellationToken.None);

This saves about ~10% of the allocations (20.164 => 17.940).

Before

After

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Issue Details

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ClientWebSocketOptions.Proxy is set by default, so the shared handler wasn't being used in the common case.
This PR adds a second handler field that's used when a default proxy is requested.
While we're changing this code, I also changed it to store an HttpMessageInvoker instead of SocketsHttpHandler.

For the following test case

usingvarcws=newClientWebSocket();awaitcws.ConnectAsync(uri,CancellationToken.None);

This saves about ~10% of the allocations (20.164 => 17.940).

Before

After

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Net
Milestone:	8.0.0

[MihaZupan]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[MihaZupan]

Outerloop failures are still #74468