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

scripts/generate_code.py

@@ -103,6 +103,7 @@ CPP_17_OPTS = NO_INCL_OPTS + [
     "--gen-object-api",
 ]
 RUST_OPTS = BASE_OPTS + ["--rust", "--gen-all", "--gen-name-strings"]
+RUST_SERIALIZE_OPTS = BASE_OPTS + ["--rust", "--gen-all", "--gen-name-strings", "--rust-serialize"]
 TS_OPTS = ["--ts", "--gen-name-strings"]
 LOBSTER_OPTS = ["--lobster"]
 SWIFT_OPTS = ["--swift", "--gen-json-emit", "--bfbs-filenames", str(tests_path)]
@@ -155,6 +156,14 @@ flatc(
     data="monsterdata_test.json",
 )
 
+flatc(
+    RUST_SERIALIZE_OPTS,
+    schema="monster_test.fbs",
+    include="include_test",
+    prefix="monster_test_serialize",
+    data="monsterdata_test.json",
+)
+
 flatc(
     options=BASE_OPTS + ["--python"],
     schema="monster_test.fbs",