[csharp] Add useVirtualForHooks option

[sbward]

When using C# OpenAPI clients with HttpClient or RestSharp from a browser via WASM (such as in Blazor WASM) it can be necessary to customize the HttpClient or RestRequest before sending it. In my case I need to invoke httpRequestMessage.SetBrowserRequestCredentials(BrowserRequestCredentials.Include) on the underlying HTTP request so that the browser's secure cookies are included with the request.

To achieve this in a backwards-compatible way I've introduced the useVirtualForHooks option for C#, which alters the definition of the InterceptRequest and InterceptResponse hooks to become public virtual methods, allowing the end-user to define custom HTTP request hooks. Technically I only need the InterceptRequest hook, but I've included InterceptResponse for consistency and because it might be useful for other situations in a browser context.

CC: @wing328 This is the result of our discussion about #20015
CC: C# technical committee (@mandrean @shibayan @Blackclaws @lucamazzanti @iBicha)

Fixes #14609

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