Implement a config based name manager and use it in Rust codegen (#7144)

* Refactor out a  class from Rust Codegen

* Convert GenerateRustModuleRootFile

* git-clang-format

* unused variable

* parenthesis

* update BUILD file

* buildifier

* Delete bfbs_gen_rust.h

* Delete bfbs_gen_rust.cpp

* Addressed some comments

* Namer::EnumVariant

* Remove do not submit; Add Namespace vector overload

* Unshadow variable

* removed redundant variables

* Warn if converting from kKeep case.

Co-authored-by: Casper Neo <cneo@google.com>

src/BUILD.bazel

@@ -73,6 +73,7 @@ cc_library(
         "idl_gen_swift.cpp",
         "idl_gen_text.cpp",
         "idl_gen_ts.cpp",
+        "namer.h",
         "util.cpp",
     ],
     hdrs = [