Reduce HttpRequestHeader enumerator allocations

[benaadams]

Reduce allocations where the request headers are always enumerated for every request like in reverse-proxy/ReverseProxy/Service/Proxy/HttpTransformer.cs

public virtual ValueTask TransformRequestAsync(HttpContext httpContext, HttpRequestMessage proxyRequest, string destinationPrefix)
{
    foreach (var header in httpContext.Request.Headers)
    {
        var headerName = header.Key;
        var headerValue = header.Value;

[halter73]

It'll be interesting to see if the reduced boxing makes an observable improvement to YARP benchmarks.

[halter73]

/azp run

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[adityamandaleeka]

/azp run

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