Implement Serialize on generated rust types (#7022)

* fix for rust build

* Rust: Implement Serialize on generated types

For debugging convenience it is really handy to be able to dump out
types as another format (ie: json). For example, if we are logging a
type to a structured logging system, or even printing it out in a
structured way to the console.

Right now we handle this by shelling out to `flatc` which is not ideal;
by implementing Serialize on the generated types we can use any of the
Serializer-implementing packages for our structured debug output.

* clang-format

* Make the flatbuffers Rust crate only have an optional dependency on the `serde` packages.

* fix warning

* fix rust test build

* Oh yeah this needs to be initialized

* fix toml syntax

* code review feedback

* rebuild test data

tests/arrays_test/my_game/example/array_struct_generated.rs

@@ -73,6 +73,7 @@ impl<'a> flatbuffers::Verifiable for ArrayStruct {
     v.in_buffer::<Self>(pos)
   }
 }
+
 impl<'a> ArrayStruct {
   #[allow(clippy::too_many_arguments)]
   pub fn new(