RequestAsync support basic auth

[766974616c79]

Using the above code, the builder returns a URL that is not supported by the HTTP client. You must use the Authorisation header to be able to use authentication.

Resolves Facepunch/sbox-issues#5798

[Copilot]

Pull request overview

This PR adds support for HTTP Basic Authentication by extracting credentials from the URI's UserInfo component and converting them into a proper Authorization header, resolving an issue where the builder returns URLs not supported by the HTTP client.

Key Changes:

• Extracts username and password from URI's UserInfo property
• Converts credentials to Base64-encoded Authorization header
• Strips credentials from the URI before creating the HTTP request

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[Copilot]

Using string replacement to remove credentials from the URI is fragile and could fail if the userInfo contains special regex characters or appears elsewhere in the URL. Use UriBuilder to properly reconstruct the URI without credentials by setting its UserName and Password properties to empty strings.

Suggested change

	uri = new Uri( requestUri.Replace( $"{userInfo}@", "" ), UriKind.Absolute );
	var uriBuilder = new UriBuilder( uri )
	{
	UserName = string.Empty,
	Password = string.Empty
	};
	uri = uriBuilder.Uri;

[Copilot]

TryAdd will silently fail if an Authorization header already exists, potentially causing authentication to fail unexpectedly. Consider either throwing an exception if the header already exists, or explicitly overwriting it with the credentials from the URI to ensure predictable behavior.

Suggested change

	headers.TryAdd( "Authorization", $"Basic {Convert.ToBase64String( bytes )}" );
	headers["Authorization"] = $"Basic {Convert.ToBase64String( bytes )}";