os: add File.read_into_ptr (#11219)

vlib/os/file.c.v

@@ -463,6 +463,12 @@ pub fn (f &File) read_from(pos u64, mut buf []byte) ?int {
 	return error('Could not read file')
 }
 
+// read_into_ptr reads at most max_size bytes from the file and writes it into ptr.
+// Returns the amount of bytes read or an error.
+pub fn (f &File) read_into_ptr(ptr &byte, max_size int) ?int {
+	return fread(ptr, 1, max_size, f.cfile)
+}
+
 // **************************** Utility  ops ***********************
 // flush writes any buffered unwritten data left in the file stream.
 pub fn (mut f File) flush() {