Replace ExternalStream with new ByteStream type (#12774)

# Description
This PR introduces a `ByteStream` type which is a `Read`-able stream of
bytes. Internally, it has an enum over three different byte stream
sources:
```rust
pub enum ByteStreamSource {
    Read(Box<dyn Read + Send + 'static>),
    File(File),
    Child(ChildProcess),
}
```

This is in comparison to the current `RawStream` type, which is an
`Iterator<Item = Vec<u8>>` and has to allocate for each read chunk.

Currently, `PipelineData::ExternalStream` serves a weird dual role where
it is either external command output or a wrapper around `RawStream`.
`ByteStream` makes this distinction more clear (via `ByteStreamSource`)
and replaces `PipelineData::ExternalStream` in this PR:
```rust
pub enum PipelineData {
    Empty,
    Value(Value, Option<PipelineMetadata>),
    ListStream(ListStream, Option<PipelineMetadata>),
    ByteStream(ByteStream, Option<PipelineMetadata>),
}
```

The PR is relatively large, but a decent amount of it is just repetitive
changes.

This PR fixes #7017, fixes #10763, and fixes #12369.

This PR also improves performance when piping external commands. Nushell
should, in most cases, have competitive pipeline throughput compared to,
e.g., bash.
| Command | Before (MB/s) | After (MB/s) | Bash (MB/s) |
| -------------------------------------------------- | -------------:|
------------:| -----------:|
| `throughput \| rg 'x'` | 3059 | 3744 | 3739 |
| `throughput \| nu --testbin relay o> /dev/null` | 3508 | 8087 | 8136 |

# User-Facing Changes
- This is a breaking change for the plugin communication protocol,
because the `ExternalStreamInfo` was replaced with `ByteStreamInfo`.
Plugins now only have to deal with a single input stream, as opposed to
the previous three streams: stdout, stderr, and exit code.
- The output of `describe` has been changed for external/byte streams.
- Temporary breaking change: `bytes starts-with` no longer works with
byte streams. This is to keep the PR smaller, and `bytes ends-with`
already does not work on byte streams.
- If a process core dumped, then instead of having a `Value::Error` in
the `exit_code` column of the output returned from `complete`, it now is
a `Value::Int` with the negation of the signal number.

# After Submitting
- Update docs and book as necessary
- Release notes (e.g., plugin protocol changes)
- Adapt/convert commands to work with byte streams (high priority is
`str length`, `bytes starts-with`, and maybe `bytes ends-with`).
- Refactor the `tee` code, Devyn has already done some work on this.

---------

Co-authored-by: Devyn Cairns <devyn.cairns@gmail.com>

tests/plugins/stream.rs

@@ -119,40 +119,40 @@ fn sum_big_stream() {
 }
 
 #[test]
-fn collect_external_accepts_list_of_string() {
+fn collect_bytes_accepts_list_of_string() {
     let actual = nu_with_plugins!(
         cwd: "tests/fixtures/formats",
         plugin: ("nu_plugin_example"),
-        "[a b] | example collect-external"
+        "[a b] | example collect-bytes"
     );
 
     assert_eq!(actual.out, "ab");
 }
 
 #[test]
-fn collect_external_accepts_list_of_binary() {
+fn collect_bytes_accepts_list_of_binary() {
     let actual = nu_with_plugins!(
         cwd: "tests/fixtures/formats",
         plugin: ("nu_plugin_example"),
-        "[0x[41] 0x[42]] | example collect-external"
+        "[0x[41] 0x[42]] | example collect-bytes"
     );
 
     assert_eq!(actual.out, "AB");
 }
 
 #[test]
-fn collect_external_produces_raw_input() {
+fn collect_bytes_produces_byte_stream() {
     let actual = nu_with_plugins!(
         cwd: "tests/fixtures/formats",
         plugin: ("nu_plugin_example"),
-        "[a b c] | example collect-external | describe"
+        "[a b c] | example collect-bytes | describe"
     );
 
-    assert_eq!(actual.out, "raw input");
+    assert_eq!(actual.out, "byte stream");
 }
 
 #[test]
-fn collect_external_big_stream() {
+fn collect_bytes_big_stream() {
     // This in particular helps to ensure that a big stream can be both read and written at the same
     // time without deadlocking
     let actual = nu_with_plugins!(
@@ -160,9 +160,8 @@ fn collect_external_big_stream() {
         plugin: ("nu_plugin_example"),
         r#"(
             seq 1 10000 |
-                to text |
-                each { into string } |
-                example collect-external |
+                each {|i| ($i | into string) ++ (char newline) } |
+                example collect-bytes |
                 lines |
                 length
         )"#