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

samples/rust_generated/my_game/sample/monster_generated.rs

@@ -205,6 +205,7 @@ impl<'a> Default for MonsterArgs<'a> {
     }
   }
 }
+
 pub struct MonsterBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,

samples/rust_generated/my_game/sample/vec_3_generated.rs

@@ -70,6 +70,7 @@ impl<'a> flatbuffers::Verifiable for Vec3 {
     v.in_buffer::<Self>(pos)
   }
 }
+
 impl<'a> Vec3 {
   #[allow(clippy::too_many_arguments)]
   pub fn new(

samples/rust_generated/my_game/sample/weapon_generated.rs

@@ -89,6 +89,7 @@ impl<'a> Default for WeaponArgs<'a> {
     }
   }
 }
+
 pub struct WeaponBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,