This should get the scheme for the request. This differs from `TSUrlSchemeGet` in that it gets the scheme even if it is not in the URL of the request. For most proxy requests, the ATS core will remove the host and scheme in the request while tracking it internally. In such a case a plugin cannot discover that information, a problem this API would fix.
If the scheme is in the request URL, return that. Otherwise return a scheme that corresponds to the internally stored scheme.
