C++ codegen of structs and unions (#2707)

* Part of #2647

### What
This declares all C++ types and their fields. A matching `#include` list
is also included. The resulting C++ builds.

Test it yourself with `cargo codegen &&
./examples/cpp/minimal/build_and_run.sh`

Sum-types are implemented via "tagged unions" approach, basically an
emulation of how Rust does it under the hood. see #2647 for details.

### Done:
* struct declarations
* enum declarations
* enum destructors
* enum move semantics
* enum static constructors
* Implicit constructors for enums
* Implicit constructor for single-fields structs (e.g. `Vec3D`)

### Still left to do:
* Arrow-serialization
* Helper functions for building archetypes (needs design)

### Things I think we should punt on:
* Copy constructors
* Copy assignment operator

### 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 [demo.rerun.io](https://demo.rerun.io/pr/2707) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2707)
- [Docs
preview](https://rerun.io/preview/pr%3Aemilk%2Fcodegen-cpp-struct/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Aemilk%2Fcodegen-cpp-struct/examples)

.clang-format

@@ -12,10 +12,12 @@ AlwaysBreakBeforeMultilineStrings: true
 BinPackArguments: false
 ColumnLimit: 100
 ContinuationIndentWidth: 4
+EmptyLineBeforeAccessModifier: LogicalBlock
 IndentWidth: 4
 IndentWrappedFunctionNames: true
 InsertTrailingCommas: Wrapped
 MaxEmptyLinesToKeep: 1
 NamespaceIndentation: All
 PointerAlignment: Left
+SeparateDefinitionBlocks: Always
 SpacesBeforeTrailingComments: 1

crates/re_types/source_hash.txt

@@ -1,4 +1,4 @@
 # This is a sha256 hash for all direct and indirect dependencies of this crate's build script.
 # It can be safely removed at anytime to force the build script to run again.
 # Check out build.rs to see how it's computed.
-d5ccdc80e148c8058d3885418f9c701fbae4a5fbb92defb9bd625ad27ed97a25
+289571cd5bcef516a43ead1678f39054c300890396d13933dc26d9294a07dffe

crates/re_types_builder/src/arrow_registry.rs

@@ -300,7 +300,7 @@ impl From<DataType> for LazyDatatype {
             DataType::Extension(name, datatype, metadata) => {
                 LazyDatatype::Extension(name, Box::new((*datatype).into()), metadata)
             }
-            _ => unimplemented!("{datatype:#?}"), // NOLINT
+            _ => unimplemented!("{datatype:#?}"),
         }
     }
 }