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.

tests/shell/pipeline/commands/external.rs

@@ -97,6 +97,16 @@ fn redirects_custom_command_external() {
     assert_eq!(actual.out, "8");
 }
 
+#[test]
+fn passes_binary_data_between_externals() {
+    let actual = nu!(cwd: "tests/fixtures/formats", r#"nu --testbin meowb sample.db | nu --testbin relay | hash sha256"#);
+
+    assert_eq!(
+        actual.out,
+        "2f5050e7eea415c1f3d80b5d93355efd15043ec9157a2bb167a9e73f2ae651f2"
+    )
+}
+
 mod it_evaluation {
     use super::nu;
     use nu_test_support::fs::Stub::{EmptyFile, FileWithContent, FileWithContentToBeTrimmed};