Make from json --objects streaming (#12949)

# Description

Makes the `from json --objects` command produce a stream, and read
lazily from an input stream to produce its output.

Also added a helper, `PipelineData::get_type()`, to make it easier to
construct a wrong type error message when matching on `PipelineData`. I
expect checking `PipelineData` for either a string value or an `Unknown`
or `String` typed `ByteStream` will be very, very common. I would have
liked to have a helper that just returns a readable stream from either,
but that would either be a bespoke enum or a `Box<dyn BufRead>`, which
feels like it wouldn't be so great for performance. So instead, taking
the approach I did here is probably better - having a function that
accepts the `impl BufRead` and matching to use it.

# User-Facing Changes

- `from json --objects` no longer collects its input, and can be used
for large datasets or streams that produce values over time.

# Tests + Formatting
All passing.

# After Submitting
- [ ] release notes

---------

Co-authored-by: Ian Manske <ian.manske@pm.me>

crates/nu-command/tests/format_conversions/json.rs

@@ -96,6 +96,32 @@ fn from_json_text_recognizing_objects_independently_to_table() {
     })
 }
 
+#[test]
+fn from_json_text_objects_is_stream() {
+    Playground::setup("filter_from_json_test_2_is_stream", |dirs, sandbox| {
+        sandbox.with_files(&[FileWithContentToBeTrimmed(
+            "katz.txt",
+            r#"
+                {"name":   "Yehuda", "rusty_luck": 1}
+                {"name": "JT", "rusty_luck": 1}
+                {"name":   "Andres", "rusty_luck": 1}
+                {"name":"GorbyPuff", "rusty_luck": 3}
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open katz.txt
+                | from json -o
+                | describe -n
+            "#
+        ));
+
+        assert_eq!(actual.out, "stream");
+    })
+}
+
 #[test]
 fn from_json_text_recognizing_objects_independently_to_table_strict() {
     Playground::setup("filter_from_json_test_2_strict", |dirs, sandbox| {