enum codegen for Python (#5319)

### What
* Part of #3384

Even includes codegen of the arrow serialization, which is a first for
Python.

Best reviewed commit-by-commit

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/5319/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5319/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/5319/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5319)
- [Docs
preview](https://rerun.io/preview/2f1d162e5f221d9c0a78b1bcff1524fec67e110c/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/2f1d162e5f221d9c0a78b1bcff1524fec67e110c/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

Author: emilk

crates/re_types/definitions/rerun/datatypes.fbs

@@ -32,5 +32,6 @@ include "./datatypes/vec2d.fbs";
 include "./datatypes/vec3d.fbs";
 include "./datatypes/vec4d.fbs";
 
+include "./testing/datatypes/enum.fbs";
 
 namespace rerun.datatypes;

crates/re_types/definitions/rerun/testing/datatypes/enum.fbs

@@ -0,0 +1,22 @@
+namespace rerun.testing.datatypes;
+
+/// A test of the enum type.
+enum EnumTest: byte {
+    /// Great film.
+    Up,
+
+    /// Feeling blue.
+    Down,
+
+    /// Correct.
+    Right,
+
+    /// It's what's remaining.
+    Left,
+
+    /// It's the only way to go.
+    Forward,
+
+    /// Baby's got it.
+    Back,
+}

crates/re_types/src/testing/datatypes/.gitattributes

@@ -293,18 +293,29 @@ pub fn collect_examples_for_api_docs<'a>(
 }
 
 pub trait StringExt {
-    fn push_text(&mut self, text: impl AsRef<str>, linefeeds: usize, indent: usize) -> &mut Self;
-    fn push_unindented_text(&mut self, text: impl AsRef<str>, linefeeds: usize) -> &mut Self;
+    fn push_indented(
+        &mut self,
+        indent_level: usize,
+        text: impl AsRef<str>,
+        linefeeds: usize,
+    ) -> &mut Self;
+
+    fn push_unindented(&mut self, text: impl AsRef<str>, linefeeds: usize) -> &mut Self;
 }
 
 impl StringExt for String {
-    fn push_text(&mut self, text: impl AsRef<str>, linefeeds: usize, indent: usize) -> &mut Self {
-        self.push_str(&indent::indent_all_by(indent, text.as_ref()));
+    fn push_indented(
+        &mut self,
+        indent_level: usize,
+        text: impl AsRef<str>,
+        linefeeds: usize,
+    ) -> &mut Self {
+        self.push_str(&indent::indent_all_by(indent_level * 4, text.as_ref()));
         self.push_str(&vec!["\n"; linefeeds].join(""));
         self
     }
 
-    fn push_unindented_text(&mut self, text: impl AsRef<str>, linefeeds: usize) -> &mut Self {
+    fn push_unindented(&mut self, text: impl AsRef<str>, linefeeds: usize) -> &mut Self {
         self.push_str(&unindent::unindent(text.as_ref()));
         self.push_str(&vec!["\n"; linefeeds].join(""));
         self