Spring Use readNBytes in StringHttpMessageConverter when contentLength is available

When the content length is known, use readNBytes on the InputStream in StringHttpMessageConverter, which avoids some extra copying and allocations.

This is essentially the same optimization as #30010.

Comment From: sbrannen

This has been merged into 6.0.x and main in 7636eecb481d9d5ec9a145ec800dec09b90743da and revised in c0506422908ef0d985c0133f7e1c1059e6cb34e1.

Thanks