Support binary data to stdin of run-external (#4984)

* Add test for passing binary data through externals

This change adds an ignored test to confirm that binary data is passed
correctly between externals to be enabled in a later commit along with
the fix.

To assist in platform agnostic testing of binary data a couple of
additional testbins were added to allow testing on `Value::Binary` inside
`ExternalStream`.

* Support binary data to stdin of run-external

Prior to this change, any pipeline producing binary data (not detected
as string) then feed into an external would be ignored due to
run-external only supporting `Value::String` on stdin.

This change adds binary stdin support for externals allowing something
like this for example:

  〉^cat /dev/urandom | ^head -c 1MiB | ^pv -b | ignore
  1.00MiB

This would previously output `0.00 B [0.00 B/s]` due to the data not
being pushed to stdin at each stage.

crates/nu-command/src/system/run_external.rs

@@ -169,12 +169,13 @@ impl ExternalCommand {
 
                             if let Ok(input) = input {
                                 for value in input.into_iter() {
-                                    if let Value::String { val, span: _ } = value {
-                                        if stdin_write.write(val.as_bytes()).is_err() {
-                                            return Ok(());
-                                        }
-                                    } else {
-                                        return Err(());
+                                    let buf = match value {
+                                        Value::String { val, .. } => val.into_bytes(),
+                                        Value::Binary { val, .. } => val,
+                                        _ => return Err(()),
+                                    };
+                                    if stdin_write.write(&buf).is_err() {
+                                        return Ok(());
                                     }
                                 }
                             }