[Rust] Implement 'object' type conversion (#6846)

* [Rust] Use serde Value for objects

This hopefully fixes the previous TODO; it at least fixes compilation
errors for the specific swagger model I'm working with.

* [Rust] Update Cargo.toml to specify versions

Letting the version float freely is scary, to say the least.

This gives it a better chance at being future-proof.

When the crate author had a recommended selector I picked that,
otherwise I went semver compatible.

* [Rust] Regenerate the example

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/RustClientCodegen.java

@@ -99,8 +99,7 @@ public class RustClientCodegen extends DefaultCodegen implements CodegenConfig {
         typeMapping.put("file", "File");
         typeMapping.put("binary", "Vec<u8>");
         typeMapping.put("ByteArray", "String");
-        // TODO  what should 'object' mapped to
-        typeMapping.put("object", "Object");
+        typeMapping.put("object", "Value");
 
         // no need for rust
         //importMapping = new HashMap<String, String>();