Write a function CountingWriter with the signature below that, given an io.Writer,
returns a new Writer that wraps the original, and a pointer to an int64 variable
that at any moment contains the number of bytes written to the new Writer.
func CountingWriter(w io.Writer) (io.Writer, *int64)We first use CountingWriter to get a Writer and then we write a string to an empty strings.Builder.
We then write a second string using the same Writer as before, therefore to the same strings.Builder.
Here we expect the count to be the length of concatenating the second string to the first.
Finally we write a third string to the same strings.Builder as before.
Except this time we use a second Writer
created by passing the original strings.Builder to CountingWriter.
At this point we want the byte count for this second Writer to be the length of the third string only.
The byte count of the first Writer should not change
because the two Writer objects are distinct instances.
The string representation of the encapsulated strings.Builder used by both Writer's
should then be the concatenation of all three strings.