RequestTransformContext.DestinationPrefix should be read-write

[TrevorDArcyEvans]

Describe the bug

commit: 527fafe

Comments in RequestTransformContext suggest that DestinationPrefix is able to be updated:

     /// <summary>
    /// The outgoing proxy request. All field are initialized except for the 'RequestUri' and optionally headers.
    /// If no value is provided then the 'RequestUri' will be initialized using the updated 'DestinationPrefix',
    /// 'Path', and 'Query' properties after the transforms have run. The headers will be copied later when
    /// applying header transforms.
    /// </summary>
    public HttpRequestMessage ProxyRequest { get; init; } = default!;

but DestinationPrefix cannot be changed:

    /// <summary>
    /// The URI prefix for the proxy request. This includes the scheme and host and can optionally include a
    /// port and path base. The 'Path' and 'Query' properties will be appended to this after the transforms have run.
    /// </summary>
    public string DestinationPrefix { get; init; } = default!;

Question
Is this a bug in the code or the comment?

[Tratcher]

HttpRequestMessage.RequestUri is what transforms can set to override DestinationPrefix.

reverse-proxy/src/ReverseProxy/Transforms/Builder/StructuredTransformer.cs

Lines 118 to 125 in 527fafe

	// Allow a transform to directly set a custom RequestUri.
	if (proxyRequest.RequestUri is null)
	{
	var queryString = transformContext.MaybeQuery?.QueryString ?? httpContext.Request.QueryString;
	proxyRequest.RequestUri = RequestUtilities.MakeDestinationAddress(
	transformContext.DestinationPrefix, transformContext.Path, queryString);
	}

I guess we could make DestinationPrefix mutable since you can already override it via RequestUri. Careful though, changing the destination prefix at this point can mess up load balancing, health checks, etc. if the proxy is attributing state from one prefix to another host.

Why do you want to change the prefix?

[TrevorDArcyEvans]

LOL! I am already overriding it via RequestUri and it is a bit fiddly. Updating DestinationPrefix is much cleaner and simpler, albeit with the riders you pointed out.

Could we make it mutable with some suitable warnings in the comments?