Fix Rust codegen for AnyType

[aidanhs]

Simply adds a type for AnyType for Rust codegen. I can see that this is present for other languages, so is really just filling a missing piece:

$ rg '"AnyType"'
modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
1451:        typeMapping.put("AnyType", "oas_any_type_not_mapped");
2082:            return "AnyType";

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPhpCodegen.java
130:        typeMapping.put("AnyType", "mixed");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java
178:        typeMapping.put("AnyType", "Object");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java
173:        typeMapping.put("AnyType", "any");
896:            ? schemas.stream().filter(schema -> !"AnyType".equals(super.getSchemaType(schema))).collect(Collectors.toList())

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartDioClientCodegen.java
92:        typeMapping.put("AnyType", "Object");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractGoCodegen.java
133:        typeMapping.put("AnyType", "interface{}");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/RustServerCodegen.java
214:        typeMapping.put("AnyType", "serde_json::Value");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java
138:        typeMapping.put("AnyType", "object");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java
142:        typeMapping.put("AnyType", "Object");

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cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05)

[wing328]

@aidanhs thanks for the PR, which looks good to me