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/monster_test/my_game/example/ability_generated.rs

@@ -69,6 +69,7 @@ impl<'a> flatbuffers::Verifiable for Ability {
     v.in_buffer::<Self>(pos)
   }
 }
+
 impl<'a> Ability {
   #[allow(clippy::too_many_arguments)]
   pub fn new(

tests/monster_test/my_game/example/monster_generated.rs

@@ -847,6 +847,7 @@ impl<'a> Default for MonsterArgs<'a> {
     }
   }
 }
+
 pub struct MonsterBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,

tests/monster_test/my_game/example/referrable_generated.rs

@@ -86,6 +86,7 @@ impl<'a> Default for ReferrableArgs {
     }
   }
 }
+
 pub struct ReferrableBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,

tests/monster_test/my_game/example/stat_generated.rs

@@ -110,6 +110,7 @@ impl<'a> Default for StatArgs<'a> {
     }
   }
 }
+
 pub struct StatBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,

tests/monster_test/my_game/example/struct_of_structs_generated.rs

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

tests/monster_test/my_game/example/test_generated.rs

@@ -69,6 +69,7 @@ impl<'a> flatbuffers::Verifiable for Test {
     v.in_buffer::<Self>(pos)
   }
 }
+
 impl<'a> Test {
   #[allow(clippy::too_many_arguments)]
   pub fn new(

tests/monster_test/my_game/example/test_simple_table_with_enum_generated.rs

@@ -76,6 +76,7 @@ impl<'a> Default for TestSimpleTableWithEnumArgs {
     }
   }
 }
+
 pub struct TestSimpleTableWithEnumBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,

tests/monster_test/my_game/example/type_aliases_generated.rs

@@ -201,6 +201,7 @@ impl<'a> Default for TypeAliasesArgs<'a> {
     }
   }
 }
+
 pub struct TypeAliasesBuilder<'a: 'b, 'b> {
   fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
   start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,

tests/monster_test/my_game/example/vec_3_generated.rs

@@ -73,6 +73,7 @@ impl<'a> flatbuffers::Verifiable for Vec3 {
     v.in_buffer::<Self>(pos)
   }
 }
+
 impl<'a> Vec3 {
   #[allow(clippy::too_many_arguments)]
   pub fn new(