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>

include/flatbuffers/util.h

@@ -701,6 +701,8 @@ enum class Case {
   kAllLower = 6,
   // the-quick-brown-fox
   kDasher = 7,
+  // THEQuiCKBr_ownFox (or whatever you want, we won't change it)
+  kKeep = 8,
 };
 
 // Convert the `input` string of case `input_case` to the specified `output_case`.